fix: gracefully handle missing manifest revisions during imports

mentioned in commit 0f45275c

mentioned in issue #649 (closed)

mentioned in merge request gitlab-org/build/CNG!982 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1724 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1725 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2523 (merged)

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

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