Make most song info optional, to support untagged music

src/mpd_now_playable/song.py

@@ -1,5 +1,7 @@
 from enum import StrEnum
+from pathlib import Path
 from typing import Protocol
+from uuid import UUID
 
 from attrs import define, field
 
@@ -13,10 +15,18 @@ class PlaybackState(StrEnum):
 @define
 class Song:
 	state: PlaybackState
-	title: str
-	artist: str
-	album: str
-	album_artist: str
+	queue_index: int
+	queue_length: int
+	file: Path
+	musicbrainz_trackid: UUID | None
+	title: str | None
+	artist: str | None
+	composer: str | None
+	album: str | None
+	album_artist: str | None
+	track: int | None
+	disc: int | None
+	genre: str | None
 	duration: float
 	elapsed: float
 	art: bytes | None = field(repr=lambda a: "<has art>" if a else "<no art>")