Empty prev hash in block header is being encoded as null

[agaffney]

We need to make sure that an empty prev hash is encoded as a zero-filled bytestring of the correct size. It can currently show up as a null.

Current:

    [
        [
            0,
            0,
            null,
            h'ae29c552228a08f3c563450b10a9f548f4602d598e4b78dd4d70edaf12ba548d',
            h'7f69d32c32041f11992dff6d6b9445ba3e0997a074e4c700b504b359f1ef55b4',
      ...
    ],

Expected:

    [
        [
            0,
            0,
            h'0000000000000000000000000000000000000000000000000000000000000000',
            h'ae29c552228a08f3c563450b10a9f548f4602d598e4b78dd4d70edaf12ba548d',
            h'7f69d32c32041f11992dff6d6b9445ba3e0997a074e4c700b504b359f1ef55b4',
      ...
    ],