fix task_service.py (#8687)

Fix the case where pages variable might be None

### What problem does this PR solve?


### Type of change

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

api/db/services/task_service.py

@@ -338,6 +338,8 @@ def queue_tasks(doc: dict, bucket: str, name: str, priority: int):
         file_bin = STORAGE_IMPL.get(bucket, name)
         do_layout = doc["parser_config"].get("layout_recognize", "DeepDOC")
         pages = PdfParser.total_page_number(doc["name"], file_bin)
+        if pages is None:
+            pages = 0
         page_size = doc["parser_config"].get("task_page_size") or 12
         if doc["parser_id"] == "paper":
             page_size = doc["parser_config"].get("task_page_size") or 22