common/archive: Fix empty content handling in lookup_content_raw

The bytes of the empty content are currently reported as missing in the archive due to an invalid test in swh.common.archive.lookup_content_raw.

See https://archive.softwareheritage.org/api/1/content/sha1_git:e69de29bb2d1d6434b8b29ae775ad8c2e48c5391/raw/

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Migrated from D4237 (view on Phabricator)