Add support for [TreatNullAs] extended attribute for reflected attributes

Source/core/svg/SVGGlyphRefElement.idl

 /*
  * Copyright (C) 2011 Leo Yang <leoyang@webkit.org>
  *
  * 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
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 [
     Conditional=SVG_FONTS
 ] interface SVGGlyphRefElement : SVGElement {
     // FIXME: Use [Reflect] after https://bugs.webkit.org/show_bug.cgi?id=64843 is fixed.
     attribute DOMString glyphRef;
-    [Reflect] attribute DOMString format;
+    [Reflect, TreatNullAs=NullString] attribute DOMString format;
     attribute float x;
     attribute float y;
     attribute float dx;
     attribute float dy;
 };
 
 SVGGlyphRefElement implements SVGURIReference;