Minor tweaks (#10987)

### What problem does this PR solve?

1. Rename identifier name
2. Fix some return statement
3. Fix some typos

### Type of change

- [x] Refactoring

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

api/db/services/connector_service.py

@@ -123,7 +123,7 @@ class SyncLogsService(CommonService):
             e = cls.query(kb_id=kb_id, connector_id=connector_id, status=TaskStatus.SCHEDULE)
             if e:
                 logging.warning(f"{kb_id}--{connector_id} has already had a scheduling sync task which is abnormal.")
-                return
+                return None
             reindex = "1" if reindex else "0"
             return cls.save(**{
                 "id": get_uuid(),
@@ -158,7 +158,7 @@ class SyncLogsService(CommonService):
     @classmethod
     def duplicate_and_parse(cls, kb, docs, tenant_id, src):
         if not docs:
-            return
+            return None
 
         class FileObj(BaseModel):
             filename: str