OCI image spec 1.1.0-rc4 allows image index to have subject(updated in this commit) and artifactType(update in this commit). Since both fields are optional, it's possible that user can push an index with subject but no artifactType like below.

flowchart
index --subject--> image

Should we enforce the requirement on both fields like: if subject is presented, artifactType MUST be presented, otherwise the index is invalid?