screen: fix meta_screen_get_monitor_for_rect for 0x0 rects

meta_screen_get_monitor_for_rect will return the monitor that
a given rect belongs in (choosing the "best" monitor based on
overlap, if there are overlapping monitors).

It doesn't work with 0x0 rects, though.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=694725