Omit recording_mbid if no data is available; fixes...

Omit recording_mbid if no data is available; fixes funkwhale/funkwhale#1310

api/funkwhale_api/contrib/listenbrainz/funkwhale_ready.py

@@ -22,11 +22,13 @@ def get_track(track):
     album = None
     additional_info = {
         "listening_from": "Funkwhale",
-        "recording_mbid": str(track.mbid),
         "tracknumber": track.position,
         "discnumber": track.disc_number,
     }
 
+    if track.mbid:
+        additional_info["recording_mbid"] = str(track.mbid)
+
     if track.album:
         if track.album.title:
             album = track.album.title