Fix for bug #23254: COMPRESS(NULL) makes all futher COMPRESS() calls on same Item return NULL

We don't set null_value to 0 in the Item_func_compress::val_str() for 
not-NULL results.

mysql-test/r/func_compress.result

@@ -79,3 +79,13 @@ uncompress(a)	uncompressed_length(a)
 NULL	NULL
 a	1
 drop table t1;
+create table t1(a blob);
+insert into t1 values ('0'), (NULL), ('0');
+select compress(a), compress(a) from t1;
+select compress(a) is null from t1;
+compress(a) is null
+0
+1
+0
+drop table t1;
+End of 4.1 tests

mysql-test/t/func_compress.test

@@ -54,4 +54,16 @@ insert into t1 values(NULL), (compress('a'));
 select uncompress(a), uncompressed_length(a) from t1;
 drop table t1;
 
-# End of 4.1 tests
+#
+# Bug #23254: problem with compress(NULL)
+#
+
+create table t1(a blob);
+insert into t1 values ('0'), (NULL), ('0');
+--disable_result_log
+select compress(a), compress(a) from t1;
+--enable_result_log
+select compress(a) is null from t1;
+drop table t1;
+
+--echo End of 4.1 tests

sql/item_strfunc.cc

@@ -2838,6 +2838,7 @@ String *Item_func_compress::val_str(String *str)
     null_value= 1;
     return 0;
   }
+  null_value= 0;
   if (res->is_empty()) return res;
 
   /*