Fixes NPE.

if the root URL isn't configured, clients won't be able to find this, so might as well not advertise at all.

core/src/main/java/hudson/DNSMultiCast.java

@@ -28,7 +28,8 @@ public class DNSMultiCast implements Closeable {
 
             Map<String,String> props = new HashMap<String, String>();
             String rootURL = hudson.getRootUrl();
-            if (rootURL!=null)
+            if (rootURL==null)  return;
+
             props.put("url", rootURL);
             try {
                 props.put("version",String.valueOf(Jenkins.getVersion()));