Fix: Nan response for retrieval component. (#6265)

### What problem does this PR solve?

#6247

### Type of change

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

agent/component/retrieval.py

         df["content"] = df["content_with_weight"]
         del df["content_with_weight"]
         logging.debug("{} {}".format(query, df))
-        return df
+        return df.dropna()