fix(handlers): serve OCI index payloads with no media type field

mentioned in commit 39ee1e36

mentioned in merge request gitlab-org/build/CNG!1336 (merged)

mentioned in issue #947 (closed)

mentioned in merge request omnibus-gitlab!6759 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2616 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2617 (merged)

mentioned in merge request gitlab-org/charts/gitlab!3026 (merged)

mentioned in merge request gitlab-development-kit!3029 (merged)