Fix a regression from ee405dd. After ee405dd, 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.