ipa_range_check: Do not fail when no trusted domain is available
When building the domain to forest root map, we need to take the case
of IPA server having no trusted domains configured at all. Do not abort
the checks, but return an empty map instead.
Part of: https://fedorahosted.org/freeipa/ticket/4137
Reviewed-By: Alexander Bokovoy <firstname.lastname@example.org>