terrafrom: Make subnets optional for instances

terraform/modules/hcloud_instance/network.tf

 resource "hcloud_server_network" "server_network" {
   server_id = element(hcloud_server.server.*.id, count.index)
   subnet_id = var.subnet
-  count = length(hcloud_server.server.*.id)
+  count = var.subnet != null ? length(hcloud_server.server.*.id) : 0
 }

terraform/modules/hcloud_instance/variables.tf

@@ -92,4 +92,5 @@ variable "volume_size" {
 variable "subnet" {
   type        = string
   description = "Id of the additional internal network"
+  default     = null
 }