Allow networking stage to be disabled

[juliocc]

This fix prevents

╷
│ Error: Invalid index
│
│   on stage-2-security.tf line 54, in module "sec-folder":
│   54:       "roles/resourcemanager.tagUser"        = [module.net-sa-rw[0].iam_email]
│     ├────────────────
│     │ module.net-sa-rw is empty tuple
│
│ The given key does not identify an element in this collection value: the collection has no elements.
╵
╷
│ Error: Invalid index
│
│   on stage-2-security.tf line 57, in module "sec-folder":
│   57:       "roles/resourcemanager.tagViewer"      = [module.net-sa-ro[0].iam_email]
│     ├────────────────
│     │ module.net-sa-ro is empty tuple

when

fast_stage_2 = {
  networking = { enabled = false }
}