Don't error out when presenting empty frames in QSGVideoItemSurface.

There's no good reason to do so. Backends can actually provide empty
frames, for example when flushing the pipeline or after stopping
playback.

Change-Id: I687c12b667e31b25e91c3201f59c52a8969d8e05
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>

src/qtmultimediaquicktools/qdeclarativevideooutput_render.cpp

@@ -444,10 +444,6 @@ void QSGVideoItemSurface::stop()
 
 bool QSGVideoItemSurface::present(const QVideoFrame &frame)
 {
-    if (!frame.isValid()) {
-        qWarning() << Q_FUNC_INFO << "I'm getting bad frames here...";
-        return false;
-    }
     m_backend->present(frame);
     return true;
 }