QPrintDialog OSX: don't crash if ApplicationModal and no parent

Task-number: QTBUG-32464
Change-Id: I5ee2741735255254c17555dfb977ce73941d3e22
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>