Fix orig fallback stage, use helper

Author: JOJ0

beetsplug/lastgenre/__init__.py

@@ -553,14 +553,7 @@ def _try_resolve_stage(
 
         # Nothing found, leave original if configured and valid.
         if genres and self.config["keep_existing"].get():
-            if isinstance(obj, library.Item):
-                # For track items, use track artist (important for compilations).
-                artist = getattr(obj, "artist", None)
-            else:
-                # For albums, prefer albumartist, fall back to artist.
-                artist = getattr(obj, "albumartist", None) or getattr(
-                    obj, "artist", None
-                )
+            artist = self._artist_for_filter(obj)
             if valid_genres := self._filter_valid(genres, artist=artist):
                 return valid_genres, "original fallback"
             # If the original genre doesn't match a whitelisted genre, check