How to add optional 2nd disk?

[LucEast]

Hello.

I use this provider to create cloud-init vm's as the following:

.
├── README.md
├── main.tf
├── modules
│   ├── cloud-init
│   │   └── cloud-init-files
│   └── vms
│       ├── cloud-init-template.yaml
│       ├── main.tf
│       └── variables.tf
└── variables.tf

I define the vm's as modules within the main.tf.
E.g.:

main.tf

terraform {
  required_providers {
    proxmox = {
      source  = "Telmate/proxmox"
      version = "3.0.1-rc3"
    }
  }
}

provider "proxmox" {
  pm_api_url          = var.pm_api_url
  pm_api_token_id     = var.pm_api_token_id
  pm_api_token_secret = var.pm_api_token_secret
  pm_tls_insecure     = true
}

module "harbor" {
  source          = "./modules/vms"
  hostname        = "harbor"
  ip_address      = "192.168.1.10/24"
  cores           = 2
  memory          = 4096
  sockets         = 1
  disk_size       = "40G"
  agent           = var.agent
  pve_host        = var.pve_host
  pve_user        = var.pve_user
  pve_private_key = var.pve_private_key
  protection      = true
  onboot          = true
}

modules/vms/main.tf

terraform {
  required_providers {
    proxmox = {
      source  = "Telmate/proxmox"
      version = "3.0.1-rc3"
    }
  }
}

resource "local_file" "cloud_init_config" {
  content = templatefile("${path.module}/cloud-init-template.yaml", {
    hostname   = var.hostname,
    ip_address = var.ip_address,
    gateway    = var.gateway,
    dns1       = var.dns1,
    dns_search = var.dns_search
  })
  filename = "${path.module}/cloud-init-${var.hostname}.yaml"
}

resource "null_resource" "cloud_init_config_files" {
  connection {
    type        = "ssh"
    user        = var.pve_user
    private_key = file(var.pve_private_key)
    host        = var.pve_host
  }

  provisioner "file" {
    source      = local_file.cloud_init_config.filename
    destination = "/tmp/user_data_vm-${var.hostname}.yaml"
  }

  provisioner "remote-exec" {
    inline = [
      "sudo mv /tmp/user_data_vm-${var.hostname}.yaml /var/lib/vz/snippets/user_data_vm-${var.hostname}.yaml",
      "sudo chown root:root /var/lib/vz/snippets/user_data_vm-${var.hostname}.yaml"
    ]
  }
}

resource "proxmox_vm_qemu" "vm" {
  name        = var.hostname
  target_node = var.target_node
  clone       = var.template_name
  full_clone  = var.full_clone
  agent       = var.agent

  # VM resources
  cores   = var.cores
  cpu     = var.cpu
  memory  = var.memory
  sockets = var.sockets

  # Network configuration

  ipconfig0    = "ip=${var.ip_address},gw=${var.gateway}"
  nameserver   = var.dns1
  searchdomain = var.dns_search

  # Cloud-Init settings
  cicustom = "user=local:snippets/user_data_vm-${var.hostname}.yaml"

  # Disk settings
  disks {
    scsi {
      scsi0 {
        disk {
          size    = var.disk_size
          storage = var.disk_storage
        }
      }
    }
    ide {
      ide2 {
        cloudinit {
          storage = var.disk_storage
        }
      }
    }
  }

  scsihw = var.scsihw

  # Network settings
  network {
    model  = var.network_model
    bridge = var.network_bridge
  }

  protection = var.protection

  kvm     = true
  balloon = 0

  # Start VM after creation
  vm_state = var.vm_state

  onboot = var.onboot
}

I now try to add an optional 2nd scsi disk for kubernetes downstream nodes but don't know how to make it truely optional.
My idea was, that it only get created when I specify a disk_size2 within the module.

Kind regards
Lucas