Fix bug in export when artist does not exist

vrobbler/apps/scrobbles/export.py

@@ -52,8 +52,9 @@ def export_scrobbles(start_date=None, end_date=None, format="AS"):
             track = scrobble.track
             track_number = 0  # TODO Add track number
             track_rating = "S"  # TODO implement ratings?
+            track_artist = track.artist or track.album.primary_artist
             row = [
-                track.album.primary_artist.name,
+                track_artist,
                 track.album.name,
                 track.title,
                 track_number,