fix: gracefully handle missing manifest revisions during imports

mentioned in commit 39ad7e7f

mentioned in issue #689 (closed)

mentioned in merge request gitlab-org/build/CNG!1030 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1804 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1805 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2588 (merged)

mentioned in merge request omnibus-gitlab!6181 (merged)

mentioned in merge request gitlab-development-kit!2656 (merged)

mentioned in issue #1319 (closed)