empty table comment should be stored in table proto as not having a comment

[Stewart Smith]

(when using statement_transform for show create table)

drizzle> create table t1 (a int) comment='ass';
Query OK, 0 rows affected (5.9 sec)

drizzle> alter table t1 comment='';
Query OK, 0 rows affected (9.6 sec)
Records: 0 Duplicates: 0 Warnings: 0

drizzle> show create table t1;
+-------+-------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
  `a` int DEFAULT NULL
) ENGINE=InnoDB COMMENT='' COLLATE = utf8_general_ci |
+-------+-------------------------------------------------------------------------------------------------+
1 row in set (0 sec)

But *should* be instead

drizzle> show create table t1;
+-------+-------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
  `a` int DEFAULT NULL
) ENGINE=InnoDB COLLATE = utf8_general_ci |
+-------+-------------------------------------------------------------------------------------------------+
1 row in set (0 sec)