PR #254 introduces null value for latestValidHash in the case when latest VALID ancestor is unknown.

This change makes {status: INVALID_BLOCK_HASH, latestValidHash: null} semantically equal to {status: "INVALID", latestValidHash: null}. Thus, INVALID_BLOCK_HASH becomes redundant.

If all CL clients support latestValidHash: null, this change doesn't introduce backward compatibility issues but would require corresponding change in Hive tests that are checking INVALID_BLOCK_HASH.