Merge branch '501761-model-versions-with-null-author-give-an-error' into 'master'

Model registry: allow model version author to be null

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171367



Merged-by: Alper Akgun <aakgun@gitlab.com>
Approved-by: Alper Akgun <aakgun@gitlab.com>
Reviewed-by: Fred de Gier <fdegier@gitlab.com>
Co-authored-by: fdegier <fdegier@gitlab.com>