This is an issue most possibly encountered mostly on staging. But technically,
the error could happen in production as snapshot_get_all_branches could return
None as well (ignored, if it happens, Snapshot.from_dict raises).
As we can mount partial dump from production, not everything exists. Typically,
here dangling snapshots from origin visits currently. This ensures we can use
staging nonetheless.
Related to T2428
https://sentry.softwareheritage.org/share/issue/4eb69ea2cc314c4ab811809ff60c358f/