Merge branch '508844-model-registry-none-for-non-existing-values-in-sidebar' into 'master'

Model registry: "None" for non-existing values in sidebar

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175816



Merged-by: Elwyn Benson <ebenson@gitlab.com>
Approved-by: Minahil Nichols <minahilnichols@gitlab.com>
Approved-by: Elwyn Benson <ebenson@gitlab.com>
Co-authored-by: Alper Akgun <aakgun@gitlab.com>