Fix: type check for chunks (#12164)

### What problem does this PR solve?

Fix: type check for chunks

### Type of change

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

rag/flow/tokenizer/tokenizer.py

@@ -105,6 +105,9 @@ class Tokenizer(ProcessBase):
 
     async def _invoke(self, **kwargs):
         try:
+            chunks = kwargs.get("chunks")
+            kwargs["chunks"] = [c for c in chunks if c is not None]
+
             from_upstream = TokenizerFromUpstream.model_validate(kwargs)
         except Exception as e:
             self.set_output("_ERROR", f"Input error: {str(e)}")