Fix a regression from ee405dd8. After ee405dd8, is_window_rect_full_screen() may compare window
rectangles against the empty rectangles of detached monitors. For example, is_window_rect_full_screen()
can incorrectly return TRUE for a (0, 0, 1, 1) rectangle because 0 <= 0 && 1 >= 0 && 0 <= 0 && 1 >= 0
is true.