db/Helpers: "list" on album artist falls back to the artist tag

NEWS

@@ -5,6 +5,7 @@ ver 0.19 (not yet released)
   - "listneighbors" lists file servers on the local network
   - "playlistadd" supports file:///
   - "idle" with unrecognized event name fails
+  - "list" on album artist falls back to the artist tag
 * database
   - proxy: forward "idle" events
   - proxy: copy "Last-Modified" from remote directories

src/db/Helpers.cxx

@@ -58,7 +58,10 @@ CollectTags(StringSet &set, TagType tag_type, const LightSong &song)
 	assert(song.tag != nullptr);
 	const Tag &tag = *song.tag;
 
-	if (!CheckUniqueTag(set, tag, tag_type))
+	if (!CheckUniqueTag(set, tag, tag_type) &&
+	    (tag_type != TAG_ALBUM_ARTIST ||
+	     /* fall back to "Artist" if no "AlbumArtist" was found */
+	     !CheckUniqueTag(set, tag, TAG_ARTIST)))
 		set.insert("");
 
 	return true;