A condition was inverted when checking, whether the user provides an artifact for comparison.

PR: MCLIRR-5
Submitted-by: Mathias Broekelmann, <mbroekelmann@googlemail.com>

git-svn-id: file:///home/tiste/MOJOHAUS-TO-GIT/SVN-MOJO-WIP/trunk/mojo/clirr-maven-plugin@4244 52ab4f32-60fc-0310-b215-8acea882cd1b