Fix Auto DevOps missing domain error handling

For some reason this value can be an empty string and we want to ensure
that we correctly fail the job before a deploy. This is particularly
problematic at the moment when coupled with
https://gitlab.com/gitlab-org/gitlab-ce/issues/54760 as the user has no
easy way to recover without destroying the cluster and starting over.

changelogs/unreleased/59546-fix-error-handling-for-missing-domain.yml

+---
+title: Fix Auto DevOps missing domain error handling
+merge_request: 26627
+author:
+type: fixed

lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml

@@ -856,7 +856,7 @@ rollout 100%:
   function check_kube_domain() {
     ensure_kube_ingress_base_domain
 
-    if [ -z ${KUBE_INGRESS_BASE_DOMAIN+x} ]; then
+    if [[ -z "$KUBE_INGRESS_BASE_DOMAIN" ]]; then
       echo "In order to deploy or use Review Apps,"
       echo "AUTO_DEVOPS_DOMAIN or KUBE_INGRESS_BASE_DOMAIN variables must be set"
       echo "From 11.8, you can set KUBE_INGRESS_BASE_DOMAIN in cluster settings"