Don't rely on MySQL column implicit value

This is related to PostgreSQL support but it's a bigger issue than that. See these pages for reference:

http://drupal.org/node/190504
http://drupal.org/node/159330
http://dev.mysql.com/doc/refman/5.0/en/data-type-defaults.html

The messaging_message_parts table includes four VARCHAR columns with no default value and set as NOT NULL. If you INSERT a row in MySQL without setting a value for one of these columns, the INSERT will still succeed as MySQL will use its implicit value for the column type, which in this case is an empty string.

INSERT INTO messaging_message_parts (type, msgkey, method, message) VALUES('notifications-digest', 'subject', 'default', '[[site-name]] Watch list update for [user]');

will succeed in MySQL but fail in PostgreSQL as the INSERT does not provide a value for the 'module' column which is defined as NOT NULL. The query should be:

INSERT INTO message_message_parts (type, msgkey, method, module, message) VALUES ('notifications-digest', 'subject', 'default', '', '[[site-name]] Watch list update for [user]');

See attached patch for fix to messaging.module.