Part of MDEV-10134 Add full support for DEFAULT

Print default values for BLOB's.
This is a part commit for automatic changes to make the real commit smaller.
All changes here are related to that we now print DEFAULT NULL for blob and
text fields, like we do for all other fields.

mysql-test/suite/innodb/r/innodb_mysql.result

@@ -1182,7 +1182,7 @@ Warning	1071	Specified key was too long; max key length is 767 bytes
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` text,
+  `a` text DEFAULT NULL,
   KEY `a` (`a`(255))
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8
 drop table t1;