Allow null values in blobs array

[wmesard]

I have a use case where each element in the blobs array is associated with the corresponding element in the layers array of the referent OCI image. However, a blob may be omitted for any particular layer. Therefore, I would like to modify the description of blob elements to explicitly state that nulls are allowed.

If this PR is accepted, I will submit a PR against oras-go to allow that.

The workaround (if the current PR is not accepted) would be to create a zero-length artifact, just so that I'll have something to stick in the array, and push to the remote. This is doable, but less elegant.

[sajayantony]

@wmesard given that the OCI working group is closer to landing a proposal - my hope is that ORAS will consume/produce the OCI artifact as the project does stand for "OCI Registry As Storage" :)

It would be good to continue the discussion here - opencontainers/image-spec#934 (comment) and gauge if there is a desire to do something like this. I think the push would be to use an annotation in the blob rather than try to map the index of the element.

[wmesard]

Closing this PR in favor of the similar comment I posted over in opencontainers/image-spec#934 (comment)