Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/core/html/HTMLFieldSetElement.idl

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 12 matching lines @@
     [Reflect] attribute DOMString name;
 
     readonly attribute DOMString type;
 
     readonly attribute HTMLCollection elements;
 
     readonly attribute boolean         willValidate;
     readonly attribute ValidityState   validity;
     readonly attribute DOMString       validationMessage;
     boolean  checkValidity();
-    void     setCustomValidity([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString error);
+    void     setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
 };