fix: set empty access_config to null (#206)

main.tf

@@ -90,7 +90,7 @@ resource "google_compute_instance_from_template" "bastion_vm" {
   network_interface {
     subnetwork         = var.subnet
     subnetwork_project = var.host_project != "" ? var.host_project : var.project
-    access_config      = var.external_ip ? var.access_config : []
+    access_config      = var.external_ip ? var.access_config : null
     network_ip         = var.network_ip
   }