don't freak out when the queue is totally empty, as can happen when the music path is invalid or empty

music-api.py

@@ -668,7 +668,7 @@ def worker():
             # Repeat_one: loop the first track
             if repeat_mode == "repeat_one":
                 with queue_lock:
-                    current_track = queue[0]
+                    current_track = queue[0] if queue else None
                 path_current = os.path.join(MUSIC_DIR, current_track)
                 set_rds(f"RT {current_track}")
                 played_ok = stream_file(path_current)
@@ -680,7 +680,7 @@ def worker():
 
             # Normal / repeat_all
             with queue_lock:
-                current_track = queue[0]
+                current_track = queue[0] if queue else None
             path_current = os.path.join(MUSIC_DIR, current_track)
             set_rds(f"RT {current_track}")