Ignore null reporters field and avoid a NPE.

The NPE can occur if a build fails during checkout and hence the run phase
never gets called. See JENKINS-10831 for an example of this happening.

maven-plugin/src/main/java/hudson/maven/MavenBuild.java

@@ -750,8 +750,11 @@ public class MavenBuild extends AbstractMavenBuild<MavenModule,MavenBuild> {
         }
 
         public void post2(BuildListener listener) throws Exception {
-            for (MavenReporter reporter : reporters)
-                reporter.end(MavenBuild.this,launcher,listener);
+            if (reporters != null) {
+                for (MavenReporter reporter : reporters) {
+                    reporter.end(MavenBuild.this,launcher,listener);
+                }
+            }
         }
 
     }