Fix inconsistent behavior of skipped_content_missing across backends.
Two fixes:
- in-mem ignored None keys
- cassandra yielded input dicts as-is instead of a dict with just the hashes
Migrated from D2662 (view on Phabricator)
Staging instance, all changes can be removed at any time
Two fixes:
Migrated from D2662 (view on Phabricator)