window: Fix meta_window_get_workspaces when a window isn't on a workspace

Since we're going to be evaluating the work area at startup now, we need
to make sure that we don't iterate over workspaces before they're assigned.
The easiest way to do this is to make sure that meta_window_get_workspaces
doesn't crash.

https://bugzilla.gnome.org/show_bug.cgi?id=643606