Improve handling of optional variables

[omus]

Terraform doesn't seem to have a way of letting CLI input variables explicitly pass in null. To work around this we'll remove null variables and just defer to the default value.

[omus]

Reproduced the original problem which was noticed:

  ╷
  │ Error: Invalid value for variable
  │ 
  │   on main.tf line 17:
  │   17: variable "optional" {
  │     ├────────────────
  │     │ var.optional is "null"
  │ 
  │ `optional` must only be `null` or `"foo"`.
  │ 
  │ This was checked by the validation rule at main.tf:21,3-13.

– https://github.com/beacon-biosignals/terraform-action/actions/runs/12751580839/job/35538984332#step:4:119