Fix: no result if metadata returns none. (#11412)

### Type of change

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

api/apps/chunk_app.py

@@ -311,8 +311,8 @@ async def retrieval_test():
                 doc_ids = None
         elif meta_data_filter.get("method") == "manual":
             doc_ids.extend(meta_filter(metas, meta_data_filter["manual"], meta_data_filter.get("logic", "and")))
-            if not doc_ids:
-                doc_ids = None
+            if meta_data_filter["manual"] and not doc_ids:
+                doc_ids = ["-999"]
 
     try:
         tenants = UserTenantService.query(user_id=current_user.id)