Allow for default profile to be evaluated.

Undefined class parameters are not the stanrad way. There
is no easy way to set undef from Foreman. Let's use empty
string '' as shortcut.

Addressing:
Error: /Stage[main]/Openscap::Xccdf::Eval/Xccdf_scan[weekly-ssg-audit]/ensure: change from absent to present failed: Could not set 'present' on ensure: No profile '' found at 60:/etc/puppet/environments/production/modules/openscap/manifests/xccdf/eval.pp