Don't prefix the masks if they are null (prevent NPE)

fix tabs

core/src/main/java/hudson/Launcher.java

@@ -652,7 +652,9 @@ public abstract class Launcher {
             @Override
             public Proc launch(ProcStarter starter) throws IOException {
                 starter.commands.addAll(0,Arrays.asList(prefix));
-                starter.masks = prefix(starter.masks);
+                if (starter.masks != null) {
+                    starter.masks = prefix(starter.masks);
+                }
                 return outer.launch(starter);
             }