mxfdemux: don't error out if VANC track only contains packets we don't handle

mxfdemux: don't error out if VANC track only contains packets we don't handle

If the VANC track does contain packets, but we skip over all packets, just
treat it the same as if there hadn't been any packets at all and send a
GAP event instead of erroring out with "Failed to handle essence element".

We would error out because when we reach the end of the loop without having
found a closed caption packet the flow return variable is still FLOW_ERROR
which is what it has been initialised to.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1518>