Prevent warning "undefined method `strip' for nil:NilClass"

After adding this module to our Puppet server, we started getting the following warning on our non-RH/CentOS/Fedora boxes:

Could not retrieve fact='ipa_version', resolution='<anonymous>': undefined method `strip' for nil:NilClass

I've rearranged the offending code such that it won't even try to set up a new ipa_version fact if the exec of yum outputs nothing.

Existing comments suggest only acting on RH/CentOS/Fedora, but I suspect the warning would still appear if IPA weren't installed on a particular system.  This will keep it quiet in that case as well.