GStreamer: fix QMediaPlayer metadata signals.

- metaDataAvailableChanged was never emitted
- metaDataChanged was not emitted when clearing the current media

Change-Id: Ide05056450171a87aeb018be7e0bdea136341946
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>

src/plugins/gstreamer/mediaplayer/qgstreamermetadataprovider.cpp

@@ -164,6 +164,11 @@ void QGstreamerMetaDataProvider::updateTags()
          }
     }
 
+    if (oldTags.isEmpty() != m_tags.isEmpty()) {
+        emit metaDataAvailableChanged(isMetaDataAvailable());
+        changed = true;
+    }
+
     if (changed)
         emit metaDataChanged();
 }