Remove dup gb2312, solve currupt error. (#5326)

### What problem does this PR solve?

#5252 
#5325

### Type of change

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

rag/llm/chat_model.py

@@ -47,6 +47,8 @@ class Base(ABC):
                 model=self.model_name,
                 messages=history,
                 **gen_conf)
+            if not response.choices:
+                return "", 0
             ans = response.choices[0].message.content.strip()
             if response.choices[0].finish_reason == "length":
                 if is_chinese(ans):