OPTIONAL field MAY be null
OPTIONAL field MAY be null

Signed-off-by: Lei Jitang <9ac444d2b5df3db1f31aa1c6462ac8e9e2bde241@huawei.com>

vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,opencontainers/image-spec,vbatts/oci-image-spec,jonboulle/image-spec