message: Call callback even if mediaStream is not present or is closed
diff: @@ -392,10 +392,14 @@ NAN_METHOD(MediaStream::enableSlideShowBelowSpatialLayer) {
  NAN_METHOD(MediaStream::getStats) {
  MediaStream* obj = Nan::ObjectWrap::Unwrap<MediaStream>(info.Holder());
  + Nan::Callback *callback = new Nan::Callback(info[0].As<Function>());
  if (!obj->me || info.Length() != 1 || obj->closed_) {
  + Local<Value> argv[] = {
  + Nan::New<v8::String>("{}").ToLocalChecked()
  + };
  + Nan::Call(*callback, 1, argv);
  return;
  }
  - Nan::Callback *callback = new Nan::Callback(info[0].As<Function>());
  AsyncQueueWorker(new StatCallWorker(callback, obj->me));
  }
  