prevent possible NPE

java.lang.NullPointerException
	at hudson.maven.MavenModuleSet$3.call(MavenModuleSet.java:763)
	at hudson.maven.MavenModuleSet$3.call(MavenModuleSet.java:761)
	at hudson.model.ItemGroupMixIn.loadChildren(ItemGroupMixIn.java:115)
	at hudson.maven.MavenModuleSet.onLoad(MavenModuleSet.java:761)
	at hudson.model.Items.load(Items.java:278)
prevent an other npe