remove duplicated key in mind map (#1809)

### What problem does this PR solve?

### Type of change

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

rag/nlp/__init__.py

@@ -483,6 +483,7 @@ def naive_merge(sections, chunk_token_num=128, delimiter="\n。；！？"):
     def add_chunk(t, pos):
         nonlocal cks, tk_nums, delimiter
         tnum = num_tokens_from_string(t)
+        if not pos: pos = ""
         if tnum < 8:
             pos = ""
         # Ensure that the length of the merged chunk does not exceed chunk_token_num