Update dialoghelper.rst

At the "Validating a Hidden Response" the example validation didn't respond anything so the $password var was NULL

Author: jdecoster

components/console/helpers/dialoghelper.rst

@@ -159,6 +159,7 @@ You can also ask and validate a hidden response::
         if (trim($value) == '') {
             throw new \Exception('The password can not be empty');
         }
+        return $value;
     };
 
     $password = $dialog->askHiddenResponseAndValidate(