Update app.py

app.py

@@ -60,7 +60,7 @@ async def get_track_download_url(track_id: str) -> str:
 
 @app.route('/track_dl', methods=['POST'])
 async def track_dl():
-    track_id = request.json.get('track_id', '')
+    track_id = request.json.get('track_id')
     dl_url = await get_track_download_url()
     result = {"url": dl_url}
     return jsonify(result)