Pass ZSTD_CONTENTSIZE_UNKNOWN
Pass ZSTD_CONTENTSIZE_UNKNOWN

Summary:
Content size 0 as unknown is deprecated.
See code comments for details.

Reviewed By: felixhandte

Differential Revision: D6521582

fbshipit-source-id: 876b94cc0123c1020b456cc8f4cd86997afe0619

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