qtdemux: Only set width/height in caps if they're non-0
qtdemux: Only set width/height in caps if they're non-0

If they are not valid, then let a downstream parser complete them.

https://bugzilla.gnome.org/show_bug.cgi?id=796878

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good