Optional dynamic blocks in terraform

block.tf

variable "gpu" {
  description = "Enable and configure node GPUs"
  type = object({
    type  = string
    count = number
  })

  default = null

  # or...
  
  default = {
    type  = "nvidia-tesla-k80"
    count = 1
  }
}

resource "google_container_node_pool" "gpu-pool" {
  name               = "gpu-pool"
  location           = "us-central1-a"
  cluster            = "kewl-cluster"
  initial_node_count = 0

  autoscaling {
    min_node_count = 0
    max_node_count = 10
  }

  node_config {
    machine_type = "n1-standard-8"

    dynamic "guest_accelerator" {
      # The '*' notation here is a quirk. 
      # For non-list objects it will wrap the item in a list, for null it returns an empty list
      for_each = var.gpu[*]
      content {
        type  = guest_accelerator.value.type
        count = guest_accelerator.value.count
      }
    }
  }
}