quartz/vmr9: Allow the aspect ratio parameters to be NULL in IVMRWindowlessControl::GetNativeVideoSize().

Please refer to the commit 45789e00 which
fixes IVMRWindowlessControl9::GetNativeVideoSize().
Signed-off-by: Akihiro Sagawa <sagawa.aki@gmail.com>
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>