Room toJson fails when room doesn't have an owner

[G-Rath]

I have a couple of rooms that don't have owners, which causes toJson to fail when called on those rooms.

While this may be a bug in HipChat, there should be a null check in toJson for when the owner is null.

[G-Rath]

This is because the get_all_rooms API endpoint doens't return the owner in it's call.

Regardless this shouldn't trigger an error.