fix(music-lab): track guard if payload.track is not None: (T1 follow-up)

Code review found: empty dict `{}` was falsy and would silently skip
add_track. Use explicit None check.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

music-lab/app/internal_router.py

@@ -48,7 +48,7 @@ def music_update(payload: UpdatePayload):
         error=payload.error,
     )
 
-    if payload.track:
+    if payload.track is not None:
         try:
             db.add_track(payload.track)
         except Exception: