WW-4331 Returns null instead of throwing exception

This allows other UnknownHandler to be called

plugins/convention/src/main/java/org/apache/struts2/convention/ConventionUnknownHandler.java

@@ -420,7 +420,7 @@ public class ConventionUnknownHandler implements UnknownHandler {
      * Not used
      */
     public Object handleUnknownActionMethod(Object action, String methodName) throws NoSuchMethodException {
-        throw null;
+        return null;
     }
 
     public static class Resource {

xwork-core/src/main/java/com/opensymphony/xwork2/UnknownHandler.java

@@ -53,8 +53,8 @@ public interface UnknownHandler {
      * @since 2.1
      * @param action The action object
      * @param methodName The method name to call
-     * @return The result returned from invoking the action method
-     * @throws NoSuchMethodException If the method cannot be found
+     * @return The result returned from invoking the action method, can return <tt>null</tt>
+     * @throws NoSuchMethodException If the method cannot be found (deprecated) - should return nunll instead
      */
 	public Object handleUnknownActionMethod(Object action, String methodName) throws NoSuchMethodException;
 }