This logic is used by CRR replication feature to BackbeatClient.putMetadata on top of a null version

(cherry picked from commit 40e271f)
(cherry picked from commit 7be2ca2)

- For backward compatibility (if isNull is undefined), add the nullVersionId field to the master update. The nullVersionId is needed for listing, retrieving, and deleting null versions.

- For the new null key implementation (if isNull is defined): add the isNull2 field and set it to true to specify that the new version is null AND has been put with a Cloudserver handling null keys (i.e., supporting S3C-7352).

- Manage scenarios in which a version is marked with the isNull attribute set to true, but without a version ID. This happens after BackbeatClient.putMetadata() is applied to a standalone null master.

(cherry picked from commit 6820444)
(cherry picked from commit 8367e8a)

(cherry picked from commit 3f20120)