indexer.metadata: Fix edge case on unknown origin
storage.origin_get can return None as origin. This adds the behavior to skip
those None origins instead of failing the indexing task.
Related https://sentry.softwareheritage.org/share/issue/8551b4a0ed694e09ad1ad7bde047e48f/
Test Plan
tox
Migrated from D2716 (view on Phabricator)