Make CharacterData.substringData() and ProcessingInstruction.target not nullable

Source/core/dom/ProcessingInstruction.idl

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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.
  */
 
 // https://dom.spec.whatwg.org/#interface-processinginstruction
 
 interface ProcessingInstruction : CharacterData {
-    // FIXME: target should not be nullable.
-    readonly attribute DOMString? target;
+    readonly attribute DOMString target;
 
     // FIXME: CharacterData.sheet is not part of any modern spec.
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=28068
     readonly attribute StyleSheet sheet;
 };