fix: modify the response of metadata in Dify retrieval api (#3076)

### What problem does this PR solve?

Modify the response of metadata in Dify retrieval api

resolve   #2914

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/sdk/dify_retrieval.py

                 "content": c["content_ltks"],
                 "score": c["similarity"],
                 "title": c["docnm_kwd"],
-                "metadata": ""
+                "metadata": {}
             })
 
         return jsonify({"records": records})