lookup_content_license: Fix crash with swh.storage.indexer v0.3.0.
Because it returns attr objects instead of dicts now.
This wasn't caught before, because test_api_content_license only runs if 'nomossa' is installed, and it usually isn't.
Second half of the fix for #2729 (closed).
Migrated from D4355 (view on Phabricator)