fix(sks): type mismatch into coalesce call for nodepools definitions

[hbollon]

On the SKS module, all optional fields to define nodepools could not be used due to a type mismatch between default_nodepool and var.nodepools in the coalesce function call.
This PR solve this but however it's now mandatory to have all fields defined even those not used (set on null) on nodepool creation. For exemple:

nodepools = {
  "is-internal-green" = {
    size            = 2
    instance_type   = "standard.large"
    description     = "Default nodepool for is-internal"
    instance_prefix = null
    disk_size       = null

    labels              = null
    taints              = null
    private_network_ids = null
  }
}