Some file format standards don't require mastering-display-info and content-light-level values to be provided. Decklink however requires the static HDR metdata for the PQ transfer function which we may not have. CTA-861-G mentions that in this case, 0 may provided as an 'unknown' value which is what we use here. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7742>