Fix api reference empty bug (#1655)

### What problem does this PR solve?

fix api reference empty bug
```
for chunk_i in answer['reference'].get('chunks',[]):
                   ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'get'
```
```
return np.array([d["relevance_score"] for d in res["results"]]), res["meta"]["tokens"]["input_tokens"]+res["meta"]["tokens"]["output_tokens"]
                                                   ~~~^^^^^^^^^^^
KeyError: 'results'
```
### Type of change

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

api/apps/api_app.py

             conv.message[-1] = {"role": "assistant", "content": ans["answer"]}
 
         def rename_field(ans):
-            for chunk_i in ans['reference'].get('chunks', []):
-                chunk_i['doc_name'] = chunk_i['docnm_kwd']
-                chunk_i.pop('docnm_kwd')
+            for chunk in ans.get('reference', []):
+                for chunk_i in chunk.get('chunks', []):
+                    chunk_i['doc_name'] = chunk_i['docnm_kwd']
+                    chunk_i.pop('docnm_kwd')
 
         def stream():
             nonlocal dia, msg, req, conv
                 API4ConversationService.append_message(conv.id, conv.to_dict())
                 break
 
-            for chunk_i in answer['reference'].get('chunks',[]):
-                chunk_i['doc_name'] = chunk_i['docnm_kwd']
-                chunk_i.pop('docnm_kwd')
-
+            rename_field(answer)
             return get_json_result(data=answer)
 
     except Exception as e:
 
         conv = conv.to_dict()
         for referenct_i in conv['reference']:
+            if referenct_i is None or len(referenct_i) == 0:
+                continue
             for chunk_i in referenct_i['chunks']:
                 if 'docnm_kwd' in chunk_i.keys():
                     chunk_i['doc_name'] = chunk_i['docnm_kwd']

rag/llm/rerank_model.py

         }
 
     def similarity(self, query: str, texts: list):
+        if len(texts) == 0:
+            return np.array([]), 0
         data = {
             "model": self.model_name,
             "query": query,