add fallback for artist Top Tracks when server returns none

backend/mediaprovider/subsonic/subsonicmediaprovider.go

@@ -248,6 +248,9 @@ func (s *subsonicMediaProvider) GetTopTracks(artist mediaprovider.Artist, count
 	if err != nil {
 		return nil, err
 	}
+	if len(tr) == 0 {
+		return helpers.GetTopTracksFallback(s, artist.ID, count)
+	}
 	return sharedutil.MapSlice(tr, toTrack), nil
 }