fix(handlers): fallback to OCI media type for manifests with no payload media type

mentioned in commit 03397be7

mentioned in merge request gitlab-org/build/CNG!730 (merged)

mentioned in merge request omnibus-gitlab!5520 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1104 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1105 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)