screencasting: Fix CI

It seems on the CI the dataFrame is null at times

autotests/integration/screencasting_test.cpp

@@ -114,7 +114,9 @@ std::optional<QImage> ScreencastingTest::oneFrameAndClose(Test::ScreencastingStr
 
     std::optional<QImage> img;
     connect(&pwStream, &PipeWireSourceStream::frameReceived, qGuiApp, [&img](const PipeWireFrame &frame) {
+        if (frame.dataFrame) {
             img = frame.dataFrame->toImage();
+        }
     });
 
     QSignalSpy spy(&pwStream, &PipeWireSourceStream::frameReceived);