fix chunk modification bug (#1011)

### What problem does this PR solve?

As title.

### Type of change

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

api/db/services/llm_service.py

@@ -82,9 +82,9 @@ class TenantLLMService(CommonService):
         if model_config: model_config = model_config.to_dict()
         if not model_config:
             if llm_type in [LLMType.EMBEDDING, LLMType.RERANK]:
-                llm = LLMService.query(llm_name=llm_name)
+                llm = LLMService.query(llm_name=llm_name if llm_name else mdlnm)
                 if llm and llm[0].fid in ["Youdao", "FastEmbed", "BAAI"]:
-                    model_config = {"llm_factory": llm[0].fid, "api_key":"", "llm_name": llm_name, "api_base": ""}
+                    model_config = {"llm_factory": llm[0].fid, "api_key":"", "llm_name": llm_name if llm_name else mdlnm, "api_base": ""}
             if not model_config:
                 if llm_name == "flag-embedding":
                     model_config = {"llm_factory": "Tongyi-Qianwen", "api_key": "",