Fix:missing embedding vector on Tokenizer (#10964)

### What problem does this PR solve?
issue:
[#10890](https://github.com/infiniflow/ragflow/issues/10890)
change:
missing embedding vector on Tokenizer
### Type of change

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

rag/flow/tokenizer/tokenizer.py

@@ -63,6 +63,8 @@ class Tokenizer(ProcessBase):
         texts = []
         for c in chunks:
             txt = ""
+            if isinstance(self._param.fields, str):
+                self._param.fields=[self._param.fields]
             for f in self._param.fields:
                 f = c.get(f)
                 if isinstance(f, str):