fix ollama issuet push (infiniflow#486)

### What problem does this PR solve?

infiniflow#477 

### Type of change

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

rag/llm/chat_model.py

@@ -153,7 +153,7 @@ def chat(self, system, history, gen_conf):
                 options=options
             )
             ans = response["message"]["content"].strip()
-            return ans, response["eval_count"] + response["prompt_eval_count"]
+            return ans, response["eval_count"] + response.get("prompt_eval_count", 0)
         except Exception as e:
             return "**ERROR**: " + str(e), 0