Refactor function name (#11210)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/svr/cache_file_svr.py

@@ -28,7 +28,7 @@ def collect():
     logging.debug(doc_locations)
     if len(doc_locations) == 0:
         time.sleep(1)
-        return
+        return None
     return doc_locations