argval: cleanup default values in ArgValidatorDict._validate_impl()
argval: cleanup default values in ArgValidatorDict._validate_impl()

- split the "if" block in two. It's easier to read to only
  have one condition and to think about them independently.

- check "self.all_missing_during_validate" before getting the
  default value.

Signed-off-by: Thomas Haller <16e26612e4159990a6b3691e92592e691ff74132@redhat.com>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network