create ROW_START/ROW_END columns NOT NULL

sql/handler.cc

@@ -6869,7 +6869,7 @@ static Create_field *vers_init_sys_field(THD *thd, const char *field_name, int f
   f->field_name.str= field_name;
   f->field_name.length= strlen(field_name);
   f->charset= system_charset_info;
-  f->flags= flags;
+  f->flags= flags | NOT_NULL_FLAG;
   if (integer)
   {
     f->set_handler(&type_handler_longlong);

sql/sql_partition.cc

@@ -3464,7 +3464,6 @@ int vers_get_partition_id(partition_info *part_info,
   DBUG_ASSERT(table->versioned());
   DBUG_ASSERT(table->vers_end_field() == row_end);
 
-  // new rows have NULL in row_end
   if (row_end->is_max() || row_end->is_null())
   {
     *part_id= vers_info->now_part->id;