monitor-manager: Fall back to minimum screen size of 640 x 480

When headless, we don't have any logical monitors to derive a screen
size from, but we can't set it to empty as that will cause issues with
the clutter stage, UI widget layout and other things. To avoid such
issues, just fall back to a 640 x 480 screen size when headless.

https://bugzilla.gnome.org/show_bug.cgi?id=730551