Set default value for col preferences; fix bug #1483978

SQL/mysql5.initial.sql

@@ -29,7 +29,7 @@ CREATE TABLE `users` (
  `created` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `last_login` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `language` varchar(5) NOT NULL DEFAULT 'en',
- `preferences` text NOT NULL,
+ `preferences` text NOT NULL DEFAULT '',
  PRIMARY KEY(`user_id`)
 ) TYPE=MYISAM CHARACTER SET ascii COLLATE ascii_general_ci;