Fix: page_size can be None error (#8603)

### What problem does this PR solve?

Issue #8602

`parser_config.task_page_size` can be defaults to `None` when dataset is
created by API. This was not handled by the `task_executor.py` code thus
`page_size` could sometimes be `None` which will cause issue in line
351.

### Type of change

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

api/db/services/task_service.py

         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)
-        page_size = doc["parser_config"].get("task_page_size", 12)
+        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", 22)
+            page_size = doc["parser_config"].get("task_page_size") or 22
         if doc["parser_id"] in ["one", "knowledge_graph"] or do_layout != "DeepDOC":
             page_size = 10 ** 9
         page_ranges = doc["parser_config"].get("pages") or [(1, 10 ** 5)]