Qt: Check if game filename is null before loading.

This is so that the error message box does not
appear if the user simply cancels loading a game.

Source/Core/DolphinQt/MainWindow.cpp

@@ -160,7 +160,9 @@ void DMainWindow::DoStartPause()
 
 void DMainWindow::OnOpen()
 {
-	StartGame(ShowFileDialog());
+	QString filename = ShowFileDialog();
+	if (!filename.isNull())
+		StartGame(filename);
 }
 
 void DMainWindow::OnPlay()