https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_subnetwork

It appears this could be supported by simply adding the send_secondary_ip_range_if_empty = true argument to the underlying resource that is created in the subnets submodule of this module, like so:resource "google_compute_subnetwork" "subnetwork" {  for_each                   = local.subnets  name                       = each.value.subnet_name  ip_cidr_range              = each.value.subnet_ip  region                     = each.value.subnet_region  private_ip_google_access   = lookup(each.value, "subnet_private_access", "false")  private_ipv6_google_access = lookup(each.value, "subnet_private_ipv6_access", null)  send_secondary_ip_range_if_empty = true  dynamic "log_config" {    for_each = coalesce(lookup(each.value, "subnet_flow_logs", null), false) ?[{      aggregation_interval = each.value.subnet_flow_logs_interval      flow_sampling        = each.value.subnet_flow_logs_sampling      metadata             = each.value.subnet_flow_logs_metadata      filter_expr          = each.value.subnet_flow_logs_filter      metadata_fields      = each.value.subnet_flow_logs_metadata_fields    }] :[]    content {      aggregation_interval = log_config.value.aggregation_interval      flow_sampling        = log_config.value.flow_sampling      metadata             = log_config.value.metadata      filter_expr          = log_config.value.filter_expr      metadata_fields      = log_config.value.metadata == "CUSTOM_METADATA" ? log_config.value.metadata_fields : null    }  }  network     = var.network_name  project     = var.project_id  description = lookup(each.value, "description", null)  dynamic "secondary_ip_range" {    for_each = contains(keys(var.secondary_ranges), each.value.subnet_name) == true ? var.secondary_ranges[each.value.subnet_name] :[]content {  range_name    = secondary_ip_range.value.range_name  ip_cidr_range = secondary_ip_range.value.ip_cidr_range}  }  purpose          = lookup(each.value, "purpose", null)  role             = lookup(each.value, "role", null)  stack_type       = lookup(each.value, "stack_type", null)  ipv6_access_type = lookup(each.value, "ipv6_access_type", null)}