package.loader: Check for snapshot id prior to reifying model object

This is an issue mostly encountered on staging. But technically, the error could happen in production as snapshot_get_all_branches could return None as well (which is currently ignored). If that happens though, Snapshot.from_dict raises.

Most proeminent in staging, because we can mount partial dump from production. Thus not everything exists. Typically, recently we mounted a partial dump of origins/origin-visits only. This means origin visits will target inexisting snapshots [1]

Thix fix ensures we can use staging nonetheless. This also improves the current implementation path so win.

Related to #2428 (closed)

• [1] https://sentry.softwareheritage.org/share/issue/4eb69ea2cc314c4ab811809ff60c358f/

Test Plan

tox

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Migrated from D3196 (view on Phabricator)