The web interface says that rootPOM can be left empty, but if you do that you
get this:

java.lang.NullPointerException
hudson.maven.MavenModuleSet.submit(MavenModuleSet.java:374)
hudson.model.Job.doConfigSubmit(Job.java:511)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:585)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:95)
org.kohsuke.stapler.Function.bindAndinvoke(Function.java:51)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:63)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:270)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:240)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:270)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:207)
org.kohsuke.stapler.Stapler.service(Stapler.java:72)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

It seems that the empty string is converted to null and then the equals method
called on it:

rootPOM = Util.fixEmpty(req.getParameter("rootPOM").trim());
if(rootPOM.equals("pom.xml")) rootPOM=null; // normalization