S3: Check node parent existance during Hierarchy Build

Found in https://arkindex.teklia.com/process/9a6c7ac0-efc8-4aaa-b98f-53c9fa51cc99/0

Failing line: https://gitlab.teklia.com/arkindex/workers/import/s3/-/blob/a30c41d21846fb06dbd78421a8623d3fd54dbc61/worker_bucket_import/worker.py#L473

Traceback

Traceback (most recent call last):
  File "/usr/local/bin/worker-bucket-import", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/local/lib/python3.11/site-packages/worker_bucket_import/worker.py", line 609, in main
    ).run()
      ^^^^^
  File "/usr/local/lib/python3.11/site-packages/worker_bucket_import/worker.py", line 590, in run
    pages_to_create = self.build_hierarchy()
                      ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/worker_bucket_import/worker.py", line 473, in build_hierarchy
    if node.is_final and node.parent.key in self.pdf_paths:
                         ^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'key'