Fix field missing issue. (#3931)

### What problem does this PR solve?

#3905
### Type of change

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

rag/svr/task_executor.py

     batch_size = 16
     tts, cnts = [], []
     for d in docs:
-        tts.append(rmSpace(d["title_tks"]))
+        tts.append(rmSpace(d.get("docnm_kwd", "Title")))
         c = "\n".join(d.get("question_kwd", []))
         if not c:
             c = d["content_with_weight"]