make INFORMATION_SCHEMA.STATISTICS.COMMENT not nullable

as it can never be null (only "" or "disabled")

mysql-test/main/show_check.result

@@ -75,7 +75,7 @@ def	information_schema	STATISTICS	STATISTICS	SUB_PART	Sub_part	8	3	0	Y	36864	0	6
 def	information_schema	STATISTICS	STATISTICS	PACKED	Packed	253	10	0	Y	4096	0	8
 def	information_schema	STATISTICS	STATISTICS	NULLABLE	Null	253	3	0	N	4097	0	8
 def	information_schema	STATISTICS	STATISTICS	INDEX_TYPE	Index_type	253	16	5	N	4097	0	8
-def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	16	0	Y	4096	0	8
+def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	16	0	N	4097	0	8
 def	information_schema	STATISTICS	STATISTICS	INDEX_COMMENT	Index_comment	253	1024	0	N	4097	0	8
 def	information_schema	STATISTICS	STATISTICS	IGNORED	Ignored	253	3	2	N	4097	0	8
 Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Ignored
@@ -661,7 +661,7 @@ def	information_schema	STATISTICS	STATISTICS	SUB_PART	Sub_part	8	3	4	Y	36864	0	6
 def	information_schema	STATISTICS	STATISTICS	PACKED	Packed	253	10	0	Y	4096	0	63
 def	information_schema	STATISTICS	STATISTICS	NULLABLE	Null	253	3	0	N	4097	0	63
 def	information_schema	STATISTICS	STATISTICS	INDEX_TYPE	Index_type	253	16	5	N	4097	0	63
-def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	16	0	Y	4096	0	63
+def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	16	0	N	4097	0	63
 def	information_schema	STATISTICS	STATISTICS	INDEX_COMMENT	Index_comment	253	1024	0	N	4097	0	63
 def	information_schema	STATISTICS	STATISTICS	IGNORED	Ignored	253	3	2	N	4097	0	63
 Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Ignored
@@ -932,7 +932,7 @@ def	information_schema	STATISTICS	STATISTICS	SUB_PART	Sub_part	8	3	0	Y	36864	0	6
 def	information_schema	STATISTICS	STATISTICS	PACKED	Packed	253	30	0	Y	4096	0	33
 def	information_schema	STATISTICS	STATISTICS	NULLABLE	Null	253	9	0	N	4097	0	33
 def	information_schema	STATISTICS	STATISTICS	INDEX_TYPE	Index_type	253	48	5	N	4097	0	33
-def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	48	0	Y	4096	0	33
+def	information_schema	STATISTICS	STATISTICS	COMMENT	Comment	253	48	0	N	4097	0	33
 def	information_schema	STATISTICS	STATISTICS	INDEX_COMMENT	Index_comment	253	3072	0	N	4097	0	33
 def	information_schema	STATISTICS	STATISTICS	IGNORED	Ignored	253	9	2	N	4097	0	33
 Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Ignored

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

@@ -396,7 +396,7 @@ def	information_schema	SQL_FUNCTIONS	FUNCTION	1	NULL	YES	varchar	64	192	NULL	NUL
 def	information_schema	STATISTICS	CARDINALITY	10	NULL	YES	bigint	NULL	NULL	19	0	NULL	NULL	NULL	bigint(21)			select		NEVER	NULL	NO	NO
 def	information_schema	STATISTICS	COLLATION	9	NULL	YES	varchar	1	3	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(1)			select		NEVER	NULL	NO	NO
 def	information_schema	STATISTICS	COLUMN_NAME	8	NULL	NO	varchar	64	192	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(64)			select		NEVER	NULL	NO	NO
-def	information_schema	STATISTICS	COMMENT	15	NULL	YES	varchar	16	48	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(16)			select		NEVER	NULL	NO	NO
+def	information_schema	STATISTICS	COMMENT	15	NULL	NO	varchar	16	48	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(16)			select		NEVER	NULL	NO	NO
 def	information_schema	STATISTICS	IGNORED	17	NULL	NO	varchar	3	9	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(3)			select		NEVER	NULL	NO	NO
 def	information_schema	STATISTICS	INDEX_COMMENT	16	NULL	NO	varchar	1024	3072	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(1024)			select		NEVER	NULL	NO	NO
 def	information_schema	STATISTICS	INDEX_NAME	6	NULL	NO	varchar	64	192	NULL	NULL	NULL	utf8mb3	utf8mb3_general_ci	varchar(64)			select		NEVER	NULL	NO	NO

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

@@ -42,7 +42,7 @@ SUB_PART	bigint(3)	YES		NULL
 PACKED	varchar(10)	YES		NULL	
 NULLABLE	varchar(3)	NO		NULL	
 INDEX_TYPE	varchar(16)	NO		NULL	
-COMMENT	varchar(16)	YES		NULL	
+COMMENT	varchar(16)	NO		NULL	
 INDEX_COMMENT	varchar(1024)	NO		NULL	
 IGNORED	varchar(3)	NO		NULL	
 SHOW CREATE TABLE information_schema.STATISTICS;
@@ -62,7 +62,7 @@ STATISTICS	CREATE TEMPORARY TABLE `STATISTICS` (
   `PACKED` varchar(10),
   `NULLABLE` varchar(3) NOT NULL,
   `INDEX_TYPE` varchar(16) NOT NULL,
-  `COMMENT` varchar(16),
+  `COMMENT` varchar(16) NOT NULL,
   `INDEX_COMMENT` varchar(1024) NOT NULL,
   `IGNORED` varchar(3) NOT NULL
 ) ENGINE=MEMORY DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_general_ci
@@ -82,7 +82,7 @@ SUB_PART	bigint(3)	YES		NULL
 PACKED	varchar(10)	YES		NULL	
 NULLABLE	varchar(3)	NO		NULL	
 INDEX_TYPE	varchar(16)	NO		NULL	
-COMMENT	varchar(16)	YES		NULL	
+COMMENT	varchar(16)	NO		NULL	
 INDEX_COMMENT	varchar(1024)	NO		NULL	
 IGNORED	varchar(3)	NO		NULL	
 SELECT table_catalog, table_schema, table_name, index_schema, index_name

sql/sql_show.cc

@@ -7359,9 +7359,6 @@ static int get_schema_stat_record(THD *thd, TABLE_LIST *tables,
         table->field[12]->store(pos, strlen(pos), cs);
         if (!show_table->s->keys_in_use.is_set(i))
           table->field[14]->store(STRING_WITH_LEN("disabled"), cs);
-        else
-          table->field[14]->store("", 0, cs);
-        table->field[14]->set_notnull();
         DBUG_ASSERT(MY_TEST(key_info->flags & HA_USES_COMMENT) ==
                    (key_info->comment.length > 0));
         if (key_info->flags & HA_USES_COMMENT)
@@ -9917,7 +9914,7 @@ ST_FIELD_INFO stat_fields_info[]=
   Column("PACKED",        Varchar(10), NULLABLE, "Packed",      OPEN_FRM_ONLY),
   Column("NULLABLE",      Varchar(3),  NOT_NULL, "Null",        OPEN_FRM_ONLY),
   Column("INDEX_TYPE",    Varchar(16), NOT_NULL, "Index_type",  OPEN_FULL_TABLE),
-  Column("COMMENT",       Varchar(16), NULLABLE, "Comment",     OPEN_FULL_TABLE),
+  Column("COMMENT",       Varchar(16), NOT_NULL, "Comment",     OPEN_FULL_TABLE),
   Column("INDEX_COMMENT", Varchar(INDEX_COMMENT_MAXLEN),
                                        NOT_NULL, "Index_comment",OPEN_FRM_ONLY),
   Column("IGNORED",      Varchar(3),  NOT_NULL, "Ignored",        OPEN_FRM_ONLY),