0.11: NPE-fix

Always ensuring that a list of repositories is existing in case its null.
Possibly this is a change in the API induced by dependency updates.

Stacktrace was:
Caused by: java.lang.NullPointerException
    at org.apache.rat.mp.RatReportMojo.getSkinArtifactFile(RatReportMojo.java:114)
    at org.apache.rat.mp.RatReportMojo.execute(RatReportMojo.java:204)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)



git-svn-id: https://svn.apache.org/repos/asf/creadur/rat/trunk@1617162 13f79535-47bb-0310-9956-ffa450edef68