Allow nil values in awx_instance_group policy attr

[covert8]

• this commit allows changing the default instance group "default" with the following

import {
  to = awx_instance_group.default
  id = 2
}

resource "awx_instance_group" "default" {
  name                       = "default"
  is_container_group         = true
  pod_spec_override          = file("instance_group.yaml")
}

Perhaps this should be more generalized in other resources but it is a start