Fix syntax if nagios_hostgroups data bag based search returns nothing

Don't write out "members empty" if nothing comes back from the search.
Instead write out a host group with nothing, which is actually correct.
 Also remove an empty line so that there is a single empty line between
each host group definition.
Merge pull request #145 from tas50/COOK-2612

COOK-2612: Fix syntax if nagios_hostgroups data bag based search returns nothing