PXP-9192 Handle generating strings in date-time format

[paulineribeyre]

Jira Ticket: PXP-9192

after this update uc-cdis/sheepdog#363, with simulated data core_metadata_collection.date = b74eb3a5e6 and errors in integration tests:

"message":"'b74eb3a5e6' is not valid under any of the given schemas:
'b74eb3a5e6' is not a 'date-time' and 'b74eb3a5e6' is not of type 'null'",
"type":"INVALID_VALUE"}],
"id":null,"type":"core_metadata_collection"

New Features

• Handle generating strings in date-time format

[github-actions]

The style in this PR agrees with black. ✔️

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