get_filtered_files_content fails with "unexpected status None for content"

I just got a report from @douardda that one of the vault cooker on a mirror calls get_filtered_files_content with "status": None in one of the items passed as argument; while cooking a directory (presumably via swh-web, so it is probably the flat directory cooker)

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Migrated from T4345 (view on Phabricator)