fix OpenAI llm return bug (#1728)

### What problem does this PR solve?

fix OpenAI llm return bug

### Type of change

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

rag/llm/chat_model.py

@@ -62,6 +62,8 @@ class Base(ABC):
                 **gen_conf)
             for resp in response:
                 if not resp.choices:continue
+                if not resp.choices[0].delta.content:
+                    resp.choices[0].delta.content = ""  
                 ans += resp.choices[0].delta.content
                 total_tokens = (
                     (