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/pipeline_operation_log_service.py

@@ -101,14 +101,14 @@ class PipelineOperationLogService(CommonService):
         ok, document = DocumentService.get_by_id(referred_document_id)
         if not ok:
             logging.warning(f"Document for referred_document_id {referred_document_id} not found")
-            return
+            return None
         DocumentService.update_progress_immediately([document.to_dict()])
         ok, document = DocumentService.get_by_id(referred_document_id)
         if not ok:
             logging.warning(f"Document for referred_document_id {referred_document_id} not found")
-            return
+            return None
         if document.progress not in [1, -1]:
-            return
+            return None
         operation_status = document.run
 
         if pipeline_id: