fix: allow ACM module to work w/o metrics sa

[jmymy]

changes here: #1529 add support for a metrics SA. However if you dont enable the metrics SA the module doesn't work (possibly only when upgrading from an older version).

│ Error: Invalid index
│
│ on .terraform/modules/acm/modules/acm/creds.tf line 112, in resource "google_project_iam_member" "acm_metrics_writer_sa_role":
│ 112: member = "serviceAccount:${google_service_account.acm_metrics_writer_sa[0].email}"
│ ├────────────────
│ │ google_service_account.acm_metrics_writer_sa is empty tuple
│
│ The given key does not identify an element in this collection value: the
│ collection has no elements.
╵
╷
│ Error: Invalid index
│
│ on .terraform/modules/acm/modules/acm/outputs.tf line 37, in output "acm_metrics_writer_sa":
│ 37: value = google_service_account.acm_metrics_writer_sa[0].email
│ ├────────────────
│ │ google_service_account.acm_metrics_writer_sa is empty tuple
│
│ The given key does not identify an element in this collection value: the
│ collection has no elements.
╵
Operation failed: failed running terraform plan (exit 1)

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

Thanks for the contribution @jmymy!