Fix qt_pixmapToWinHICON() not to return invalid HICON for null pixmaps

Change-Id: Ieda923bf27514a3daa8a5de7836511ab32346260
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>