rtprtxsend: don't process or warn if no map is set
rtprtxsend: don't process or warn if no map is set

This makes it more gentle when doing "pass-through"

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