Fix https://issues.apache.org/jira/browse/VELTOOLS-191

Don't NPE when no javascript action methods can be found.
diff --git a/velocity-tools-struts/src/main/java/org/apache/velocity/tools/struts/ValidatorTool.java b/velocity-tools-struts/src/main/java/org/apache/velocity/tools/struts/ValidatorTool.java
index ec8c17d..8caa7ee 100644
--- a/velocity-tools-struts/src/main/java/org/apache/velocity/tools/struts/ValidatorTool.java
+++ b/velocity-tools-struts/src/main/java/org/apache/velocity/tools/struts/ValidatorTool.java
@@ -721,7 +721,11 @@
             }
             methods.append("(form)");
         }
-        return methods.toString();
+        if(null == methods) {
+            return null;
+        } else {
+            return methods.toString();
+        }
     }