resolve issure to call ZH?IPUAI (#277)

### What problem does this PR solve?

Issue link:#265

### Type of change

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

api/apps/conversation_app.py

@@ -360,7 +360,10 @@ def use_sql(question, field_map, tenant_id, chat_mdl):
             "|" for r in tbl["rows"]]
     if not docid_idx or not docnm_idx:
         chat_logger.warning("SQL missing field: " + sql)
-        return "\n".join([clmns, line, "\n".join(rows)]), []
+        return {
+            "answer": "\n".join([clmns, line, rows]),
+            "reference": {"chunks": [], "doc_aggs": []}
+        }
 
     rows = "\n".join([r + f" ##{ii}$$ |" for ii, r in enumerate(rows)])
     rows = re.sub(r"T[0-9]{2}:[0-9]{2}:[0-9]{2}(\.[0-9]+Z)?\|", "|", rows)