title: Return empty action or observable in effect (angular redux/ngrx)
body: <p>I'm uploading a file to server and I would like to track upload progress without spamming with many actions (on each http event). Is it possible to return something like null or empty observable in some cases? Because I want to dispatch an action only in two cases - success or failure. And now I must return an action in each http event type and it's becoming a spam then..</p>
  
  <pre><code>@Effect()
  public createMediaItem: Observable&lt;Action&gt; = this._actions
      .pipe(
          ofType(MediaItemActionTypes.MediaItemCreateAction),
          map((action: MediaItemCreateAction) =&gt; action),
          switchMap(action =&gt; {
              return this._uploadService.createMediaItem(action.formData)
                  .pipe(
                      map((event: HttpEvent&lt;HttpEventType&gt;) =&gt; {
                          switch (event.type) {
                              case HttpEventType.UploadProgress: {
                                  console.log('ignore');
                                  action.fileItem.progress = Math.round(event.loaded / event.total * 100);
                                  break;
                                  // return;
                              }
  
                              case HttpEventType.Response: {
                                  return new MediaItemCreateSuccessAction();
                              }
                              default:
                                  break;
                              // return;
                          }
                      })
                  );
          })
      );
  </code></pre>