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assets/origin-save: Fix handling of null visit dates in requests list

Description

assets/origin-save: Fix handling of null visit dates in requests list

Summary:
When a save request task succeeded but the corresponding visit is not yet available
in PostgreSQL database (replication lag), a null visit date can be returned so
handle that special case.

Closes T2557

Reviewers: Reviewers

Maniphest Tasks: T2557

Differential Revision: https://forge.softwareheritage.org/D3873