Fix:ERROR 20 Method rag.nlp.search.Dealer.search() parameter highlight="None" violates type hint bool | list, as <class "builtins.NoneType"> "None" not list or bool. (#10743)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/10733

### Type of change

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

rag/nlp/search.py

@@ -72,9 +72,12 @@ class Dealer:
     def search(self, req, idx_names: str | list[str],
                kb_ids: list[str],
                emb_mdl=None,
-               highlight: bool | list = False,
+               highlight: bool | list | None = None,
                rank_feature: dict | None = None
                ):
+        if highlight is None:
+            highlight = False
+
         filters = self.get_filters(req)
         orderBy = OrderByExpr()