Fix invalid user/group checks bug
Fix invalid user/group checks bug

The existence of output is checked for both of the invalid user/group
tasks, but the *length* of the output isn't checked. This patch
fixes that bug and only displays output when there is output to
display.

Closes-Bug: 1650113
Change-Id: I661006e6ee5c01505c1801d7d55c3823f3632ddd

major/openstack-ansible-security,major/openstack-ansible-security