Browsing snapshot with only releases does not show anything in the webapp

After deployment [1], we are no longer able to browse the new package loader snapshot:

Even though those are fine if we browse them through the api [2].

Some more irc information:

16:15 <+vlorentz> anlambert: We think we found an issue in swh-web: https://forge.softwareheritage.org/source/swh-web/browse/master/swh/web/browse/snapshot_context.py$562 returns None when the HEAD branch is a Release (or alias to Release) instead of a Revision (or alias to Revision)
16:16 <+vlorentz> so we get stuff like this on all snapshots generated by package loaders >= 1.0.0: https://webapp.staging.swh.network/browse/origin/directory/?origin_url=https://www.npmjs.com/package/chloe
16:17 <+vlorentz> even though they work fine via the API
16:18 <+ardumont> context: in staging, we "released" the "make the loader package build synthetic release (and no longer revision ones)"
16:18 <+ardumont> (well loaders)
16:19 <+ardumont> swh/meta$1206 for some dataset to reproduce
16:19 -- Notice(swhbot): swh/meta$1206 (author: ardumont):  <https://forge.softwareheritage.org/swh/meta$1206>

• [1] swh/infra/sysadm-environment#3722 (closed)

• [2] swh/meta$1206

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Migrated from T3724 (view on Phabricator)