title: Sonar / findbugs fails when root project doesn't contains any classes
body: <p>In a Java project, I'm using Sonar with Maven and it's work fine with "sonar way" profile. But when switching to the "Sonar way with Findbugs" profile, it fails :</p>
  
  <p><em>"Can not execute Sonar: Can not execute Findbugs: Findbugs needs sources to be compiled. Please build project before executing sonar and check the location of compiled classes."</em></p>
  
  <p>The project is correctly build before the execution of Sonar. But the root module / project doesn't contains any classes. So target/classes is empty</p>
  
  <p>My project is composed of several modules and a root pom.xml</p>
  
  <p>Project</p>
  
  <p>|<em>_</em> Module 1</p>
  
  <p>|<em>_</em> Module 2</p>
  
  <p>|<em>_</em> Module 3</p>
  
  <p>pom.xml</p>
  
  <p>Sonar is running fine for every modules and try to analyze the root project but this one doesn't contains any classes so it fails.</p>
  
  <p>Is there a way to exclude the root project / module (exclude root module is not possible) or tell sonar to only generate a warning in this case or another solution?</p>
  
  <p>Thanks in advance.</p>