fix(web-player): follow the API types where they are nullable

The checked-in schema was stale and typed artist, description and externalId
as always present, so the code trusted values the API returns as null. The
regenerated schema tells the truth; the API-shaped types now admit null and it
is normalised on the way into the player, where the state stays non-null.

web-player/src/lib/tracks.ts

@@ -19,7 +19,7 @@ export function targetTitle(target: TrackTarget): string {
 }
 
 export function targetArtist(target: TrackTarget): string | undefined {
-	return target.kind === 'youtube' ? target.song.artist : target.track.artist;
+	return (target.kind === 'youtube' ? target.song.artist : target.track.artist) ?? undefined;
 }
 
 export function targetExplicit(target: TrackTarget): boolean {