It exists some cases (nix and guix origins for instance) when a snapshot
branch targets a revision that is missing in the archive.
In that case, SWHID for currently browsed directory is missing as such
object does not exist.
So ensure to check SWHID validity when rendering show-metadata template.
See https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https://guix.gnu.org/sources.json
while being logged as staff.