Add support for phantom files

[hannes-ucsc]

Currently we get

[WARNING] 2022-03-18T00:14:22.172Z 6a94fe05-6c5c-5763-9e88-a4a8bca5fee3 Worker failed to handle message {'catalog': 'lm2-it', 'action': 'add', 'notification': {'source': {'id': 'f9c9ee4f-7e16-411b-b127-781930dc04dd', 'spec': 'tdr:datarepo-df6004c2:snapshot/lungmap_prod_f899709cae2c4bb988f0131142e6c7ec__20220310_20220311_lm2:/0'}, 'transaction_id': '477c950b-6333-4bdf-a7e4-2bc37e1f690d', 'match': {'bundle_uuid': '31244706-60f8-3043-8db7-a39fd7081139', 'bundle_version': '2022-03-11T17:13:52.129998Z'}}}.
Traceback (most recent call last):
File "/var/task/azul/indexer/index_controller.py", line 174, in contribute
contributions = self.transform(catalog, notification, delete)
File "/var/task/azul/indexer/index_controller.py", line 207, in transform
bundle = service.fetch_bundle(catalog, source, bundle_uuid, bundle_version)
File "/var/task/azul/indexer/index_service.py", line 182, in fetch_bundle
return plugin.fetch_bundle(bundle_fqid)
File "/var/task/azul/plugins/repository/tdr/__init__.py", line 246, in fetch_bundle
bundle = self._emulate_bundle(bundle_fqid)
File "/var/task/azul/plugins/repository/tdr/__init__.py", line 331, in _emulate_bundle
bundle.add_entity(entity_key=f'{entity_type}_{i}.json',
File "/var/task/azul/plugins/repository/tdr/__init__.py", line 653, in add_entity
drs_path=self._parse_file_id_column(entity_row['file_id']))
File "/var/task/azul/plugins/repository/tdr/__init__.py", line 717, in _parse_file_id_column
reject(file_id is None)
File "/var/task/azul/__init__.py", line 1292, in reject
raise exception(*args)
azul.RequirementError

Phantom files are defined as files with no data file object in the staging area and file_descriptor.drs_uri set to null.

https://github.com/HumanCellAtlas/dcp2/pull/55/files
https://github.com/HumanCellAtlas/metadata-schema/pull/1437/files

They will cause the TDR file_id column to be null. We should allow that if, and only if the file_descriptor column contains JSON whose drs_uri property is also null. If the property is not set, or if it is set to a value other than null and RequirementError should be raised. An unset property is invalid, support for non-null values will be added in #3631.

Downstream from the indexer, the resulting None values in drs_path in our index need to be handled gracefully.