cleanup: VARIABLE_VALUE column should be NOT NULL

it can never be set to NULL, so it should
not use MY_I_S_MAYBE_NULL flag either

mysql-test/r/show_check.result

@@ -101,19 +101,19 @@ drop table t1;
 show variables like "wait_timeout%";
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
 def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_NAME	Variable_name	253	64	12	N	1	0	8
-def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	5	Y	0	0	8
+def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	5	N	1	0	8
 Variable_name	Value
 wait_timeout	28800
 show variables like "WAIT_timeout%";
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
 def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_NAME	Variable_name	253	64	12	N	1	0	8
-def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	5	Y	0	0	8
+def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	5	N	1	0	8
 Variable_name	Value
 wait_timeout	28800
 show variables like "this_doesn't_exists%";
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
 def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_NAME	Variable_name	253	64	0	N	1	0	8
-def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	0	Y	0	0	8
+def	information_schema	SESSION_VARIABLES	SESSION_VARIABLES	VARIABLE_VALUE	Value	253	1024	0	N	1	0	8
 Variable_name	Value
 show table status from test like "this_doesn't_exists%";
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr

mysql-test/suite/funcs_1/r/is_columns_is.result

@@ -152,9 +152,9 @@ def	information_schema	FILES	UPDATE_COUNT	13	NULL	YES	bigint	NULL	NULL	19	0	NULL
 def	information_schema	FILES	UPDATE_TIME	34	NULL	YES	datetime	NULL	NULL	NULL	NULL	0	NULL	NULL	datetime			select	
 def	information_schema	FILES	VERSION	25	NULL	YES	bigint	NULL	NULL	20	0	NULL	NULL	NULL	bigint(21) unsigned			select	
 def	information_schema	GLOBAL_STATUS	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	
-def	information_schema	GLOBAL_STATUS	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
+def	information_schema	GLOBAL_STATUS	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
 def	information_schema	GLOBAL_VARIABLES	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	
-def	information_schema	GLOBAL_VARIABLES	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
+def	information_schema	GLOBAL_VARIABLES	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
 def	information_schema	INDEX_STATISTICS	INDEX_NAME	3		NO	varchar	192	576	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(192)			select	
 def	information_schema	INDEX_STATISTICS	ROWS_READ	4	0	NO	bigint	NULL	NULL	19	0	NULL	NULL	NULL	bigint(21)			select	
 def	information_schema	INDEX_STATISTICS	TABLE_NAME	2		NO	varchar	192	576	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(192)			select	
@@ -305,9 +305,9 @@ def	information_schema	SCHEMA_PRIVILEGES	PRIVILEGE_TYPE	4		NO	varchar	64	192	NUL
 def	information_schema	SCHEMA_PRIVILEGES	TABLE_CATALOG	2		NO	varchar	512	1536	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(512)			select	
 def	information_schema	SCHEMA_PRIVILEGES	TABLE_SCHEMA	3		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	
 def	information_schema	SESSION_STATUS	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	
-def	information_schema	SESSION_STATUS	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
+def	information_schema	SESSION_STATUS	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
 def	information_schema	SESSION_VARIABLES	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	
-def	information_schema	SESSION_VARIABLES	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
+def	information_schema	SESSION_VARIABLES	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)			select	
 def	information_schema	STATISTICS	CARDINALITY	10	NULL	YES	bigint	NULL	NULL	19	0	NULL	NULL	NULL	bigint(21)			select	
 def	information_schema	STATISTICS	COLLATION	9	NULL	YES	varchar	1	3	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1)			select	
 def	information_schema	STATISTICS	COLUMN_NAME	8		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)			select	

mysql-test/suite/funcs_1/r/is_columns_is_embedded.result

@@ -152,9 +152,9 @@ def	information_schema	FILES	UPDATE_COUNT	13	NULL	YES	bigint	NULL	NULL	19	0	NULL
 def	information_schema	FILES	UPDATE_TIME	34	NULL	YES	datetime	NULL	NULL	NULL	NULL	0	NULL	NULL	datetime				
 def	information_schema	FILES	VERSION	25	NULL	YES	bigint	NULL	NULL	20	0	NULL	NULL	NULL	bigint(21) unsigned				
 def	information_schema	GLOBAL_STATUS	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				
-def	information_schema	GLOBAL_STATUS	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
+def	information_schema	GLOBAL_STATUS	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
 def	information_schema	GLOBAL_VARIABLES	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				
-def	information_schema	GLOBAL_VARIABLES	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
+def	information_schema	GLOBAL_VARIABLES	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
 def	information_schema	INDEX_STATISTICS	INDEX_NAME	3		NO	varchar	192	576	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(192)				
 def	information_schema	INDEX_STATISTICS	ROWS_READ	4	0	NO	bigint	NULL	NULL	19	0	NULL	NULL	NULL	bigint(21)				
 def	information_schema	INDEX_STATISTICS	TABLE_NAME	2		NO	varchar	192	576	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(192)				
@@ -305,9 +305,9 @@ def	information_schema	SCHEMA_PRIVILEGES	PRIVILEGE_TYPE	4		NO	varchar	64	192	NUL
 def	information_schema	SCHEMA_PRIVILEGES	TABLE_CATALOG	2		NO	varchar	512	1536	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(512)				
 def	information_schema	SCHEMA_PRIVILEGES	TABLE_SCHEMA	3		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				
 def	information_schema	SESSION_STATUS	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				
-def	information_schema	SESSION_STATUS	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
+def	information_schema	SESSION_STATUS	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
 def	information_schema	SESSION_VARIABLES	VARIABLE_NAME	1		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				
-def	information_schema	SESSION_VARIABLES	VARIABLE_VALUE	2	NULL	YES	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
+def	information_schema	SESSION_VARIABLES	VARIABLE_VALUE	2		NO	varchar	1024	3072	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1024)				
 def	information_schema	STATISTICS	CARDINALITY	10	NULL	YES	bigint	NULL	NULL	19	0	NULL	NULL	NULL	bigint(21)				
 def	information_schema	STATISTICS	COLLATION	9	NULL	YES	varchar	1	3	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(1)				
 def	information_schema	STATISTICS	COLUMN_NAME	8		NO	varchar	64	192	NULL	NULL	NULL	utf8	utf8_general_ci	varchar(64)				

sql/sql_show.cc

@@ -8850,7 +8850,7 @@ ST_FIELD_INFO variables_fields_info[]=
 {
   {"VARIABLE_NAME", 64, MYSQL_TYPE_STRING, 0, 0, "Variable_name",
    SKIP_OPEN_TABLE},
-  {"VARIABLE_VALUE", 1024, MYSQL_TYPE_STRING, 0, 1, "Value", SKIP_OPEN_TABLE},
+  {"VARIABLE_VALUE", 1024, MYSQL_TYPE_STRING, 0, 0, "Value", SKIP_OPEN_TABLE},
   {0, 0, MYSQL_TYPE_STRING, 0, 0, 0, SKIP_OPEN_TABLE}
 };