fix gb2312 encoding issue (#394)

### What problem does this PR solve?

Issue link:#384
### Type of change

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

rag/nlp/search.py

@@ -237,7 +237,7 @@ class Dealer:
             pieces_.append(t)
         es_logger.info("{} => {}".format(answer, pieces_))
         if not pieces_:
-            return answer
+            return answer, set([])
 
         ans_v, _ = embd_mdl.encode(pieces_)
         assert len(ans_v[0]) == len(chunk_v[0]), "The dimension of query and chunk do not match: {} vs. {}".format(