Fix es get NotFoundError (#3659)

### What problem does this PR solve?

Fix es get NotFoundError

### Type of change

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

rag/utils/es_conn.py

                                   id=chunkId, source=True, )
                 if str(res.get("timed_out", "")).lower() == "true":
                     raise Exception("Es Timeout.")
-                if not res.get("found"):
-                    return None
                 chunk = res["_source"]
                 chunk["id"] = chunkId
                 return chunk
+            except NotFoundError:
+                return None
             except Exception as e:
                 logging.exception(f"ESConnection.get({chunkId}) got exception")
                 if str(e).find("Timeout") > 0: