Emit a final empty AudioVisualizerEvent after track stops (#701)

* Emit a final empty AudioVisualizerEvent after track stops

* fmt

Author: bcherry

lib/src/track/local/local.dart

@@ -101,7 +101,12 @@ mixin AudioTrack on Track {
     if (_eventChannel == null) {
       return;
     }
+
     await Native.stopVisualizer(mediaStreamTrack.id!);
+    events.emit(AudioVisualizerEvent(
+      track: this,
+      event: [],
+    ));
     await _streamSubscription?.cancel();
     _streamSubscription = null;
     _eventChannel = null;