When the last user exits a room, show 'no users' to remaining room owners

When the last non-room-owner user exits a room, the users menu does not currently update to show "No users".

![jabbr-room-last-user-leaves](https://f.cloud.github.com/assets/1271535/306967/93ecad32-96a7-11e2-8e5f-0e3cd7785dc3.png)

This functionality does work when the last owner leaves.