Description field defined without default and 'not null', fails on insert

Using PostgreSQL it is not possible to add any mailing lists because the initial INSERT is done without the description field, the description field is set "not null", and has no default value.

An error similar to the following is returned:

	•	warning: pg_query(): Query failed: ERROR: null value in column "description" violates not-null constraint in /usr/share/drupal6/includes/database.pgsql.inc on line 138.
	•	user warning: query: INSERT INTO mailman_lists (name, command, admin, web, webarch) VALUES ('MyList-l', 'MyList-l-request@example.org', 'MyList-l-admin@example.org', 'https://www.example.org/mailman/MyList', '') in /etc/drupal/6/sites/all/modules/mailman_manager/mailman_manager.module on line 392.

The attached patch sets the default to an empty string.