clutter/frame-clock: Notify on all empty frames

So that we maintain a perfectly balanced number of callbacks:

  dispatch == notify_ready + notify_presented

Otherwise you can't put any useful logic inside notify_ready and be sure
you're handling all the empty frames.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3961>

clutter/clutter/clutter-frame-clock.c

@@ -1003,8 +1003,7 @@ clutter_frame_clock_dispatch (ClutterFrameClock *frame_clock,
           frame_clock->state = CLUTTER_FRAME_CLOCK_STATE_PENDING_PRESENTED;
           break;
         case CLUTTER_FRAME_RESULT_IDLE:
-          frame_clock->state = CLUTTER_FRAME_CLOCK_STATE_IDLE;
-          maybe_reschedule_update (frame_clock);
+          clutter_frame_clock_notify_ready (frame_clock);
           break;
         }
       break;