I noticed that when trying to query or browse a non-existent release id, for instance
https://archive.softwareheritage.org/browse/release/c66646b7256b9417738cd4b8d4f0d0ac271df93e/
https://archive.softwareheritage.org/api/1/release/c66646b7256b9417738cd4b8d4f0d0ac271df93e/,
no exception were raised and thus errors appear.
The issue comes from the storage layer who does not yield None when trying
to retrieve a non-existent release.
That diff implements the same behaviour as in Storage.revision_get, ensuring None
is returned when querying a non-existent release.