No domain provided in 3-ingress-gke results in error

As mentioned in https://github.com/GoogleCloudPlatform/core-solution-services#add-a-http-load-balancer-with-dns-domain, DNS domain can be left blank.

However, providing no domain causes the subsequent apply (sb infra apply 3-gke-ingress) to fail with this error:

`
│ Error: Error creating ManagedSslCertificate: googleapi: Error 400: Invalid value for field 'resource.managed.domains[0]': 'None'. Invalid domain name specified., invalid
│
│   with module.ingress_gce.google_compute_managed_ssl_certificate.managed_certificate,
│   on ../../modules/ingress_gce/main.tf line 46, in resource "google_compute_managed_ssl_certificate" "managed_certificate":
│   46: resource "google_compute_managed_ssl_certificate" "managed_certificate" {
│
╵
Error when running command:  terraform apply   (working_dir=./terraform/stages/3-gke-ingress)
`