meta/stage: Check if the swap region is not empty before transforming it

As cairo_region_create_rectangles would return NULL if n_rects=0 but the upcoming
commits which port to MtkRegion disallows that

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3292>

src/backends/meta-stage-impl.c

@@ -701,7 +701,8 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
     }
 
   if (clutter_stage_view_get_onscreen (stage_view) !=
-      clutter_stage_view_get_framebuffer (stage_view))
+      clutter_stage_view_get_framebuffer (stage_view) &&
+      cairo_region_num_rectangles (swap_region) != 0)
     {
       cairo_region_t *transformed_swap_region;