[cassandra] webapp error: "KeyError: 'checksums'"

Another error (like #4863 (closed)) where cassandra backend behaves differently from the postgres one.

URL for cassandra backend => 500: https://webapp-cassandra.internal.softwareheritage.org/browse/origin/directory/?origin_url=https%3A%2F%2Fgitlab.cri.epita.fr%2Fcri%2Fdocumentation%2Fepita-doc.git

URL for postgres backend => 200: https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https%3A%2F%2Fgitlab.cri.epita.fr%2Fcri%2Fdocumentation%2Fepita-doc.git

Sentry Issue: SWH-WEBAPP-4NK

KeyError: 'checksums'
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "swh/web/browse/views/origin.py", line 37, in origin_directory_browse
    return browse_snapshot_directory(
  File "swh/web/browse/snapshot_context.py", line 883, in browse_snapshot_directory
    readmes[f["name"]] = f["checksums"]["sha1"]