Error: Iteration over null value

on .terraform/modules/htcondor_access.htcondor_ap/modules/mig/main.tf line 83, in resource "google_compute_region_instance_group_manager" "mig":
 83:     for_each = [for static_ip in var.stateful_ips : static_ip if static_ip["is_external"] == true]

 var.stateful_ips is null
A null value cannot be used as the collection in a 'for' expression.

  stateful_ips = var.enable_public_ips ? [{
    interface_name = "nic0"
    delete_rule    = "ON_PERMANENT_INSTANCE_DELETION"
    is_external    = true
  }] : null # remplace null with []