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