fix(storage): return ErrManifestEmpty when zero-lenth manifest content is encountered

mentioned in merge request gitlab-org/build/CNG!723 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1073 (merged)

mentioned in merge request omnibus-gitlab!5506 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1086 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1090 (merged)

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