AP_Menu: Do not compare delegate to NULL

Use the simpler "if (delegate_name)" since it allows simpler
implementation in the class, i.e. the bool operator rather than having
to compare to another object.

libraries/AP_Menu/AP_Menu.cpp

@@ -181,7 +181,7 @@ Menu::run(void)
     for (;;) {
 
         // run the pre-prompt function, if one is defined
-        if (NULL != _ppfunc) {
+        if (_ppfunc) {
             if (!_ppfunc())
                 return;
             _display_prompt();