Comment 2 for bug 919298

Fixed a trivial crash when no user list:
http://bazaar.launchpad.net/~unity-greeter-team/unity-greeter/trunk/revision/331

But there's still an odd entry box shown without text when this occurs. We probably want to show a username entry when the list is empty, and not when there are users. (and make sure to handle user accounts being added removed, e.g. deleting the single user account should switch to a username entry, then adding a new one should return to the list).