pom.siteDirectory mustn't be empty because in the maven-plugins root project we have :
maven.site.stage.directory=/home/brett/public_html/maven-stage-site/${pom.siteDirectory.substring(22)}
git-svn-id: https://svn.apache.org/repos/asf/maven/maven-1/plugins/trunk@264656 13f79535-47bb-0310-9956-ffa450edef68