Original toString would always throw a NPE.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@739577 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/property/NullReturn.java

@@ -33,6 +33,6 @@ public final class NullReturn {
      * {@inheritDoc}
      */
     public String toString() {
-        return String.valueOf(null);
+        return "null";
     }
 }