fix: skip empty manifest during (pre)import

mentioned in commit 999aad0a

mentioned in issue #634 (closed)

mentioned in merge request gitlab-org/build/CNG!951 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1639 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1648 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2482 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1651 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1668 (merged)

mentioned in merge request omnibus-gitlab!6064 (merged)

mentioned in merge request gitlab-development-kit!2503 (merged)