Api mailing create should work without logged in user (eg. drush)

The database field created_id is not compulsory, and is set to NULL on contact.delete. However the field is compulsory through the api.

I think it makes sense to allow mailings to be created with a NULL created_id (e.g via drush with no user set)

I acknowledge I could work around this by using the -u switch with drush, but I feel like the logic of the field being optional at the api level given it is optional at the DB level is correct & hence I should fix it there