Add [TreatNullAs=EmptyString] to innerText

This matches WebKit/Chromium but not Gecko/IE. Treating null as
empty string is consistent with textContent and innerHTML.

source

@@ -9662,7 +9662,7 @@ interface <dfn>HTMLElement</dfn> : <span>Element</span> {
   [<span>CEReactions</span>] attribute boolean <span data-x="dom-spellcheck">spellcheck</span>;
   void <span data-x="dom-forceSpellCheck">forceSpellCheck</span>();
 
-  [<span>CEReactions</span>] attribute DOMString <span data-x="dom-innerText">innerText</span>;
+  [<span>CEReactions</span>, TreatNullAs=EmptyString] attribute DOMString <span data-x="dom-innerText">innerText</span>;
 };
 <span>HTMLElement</span> implements <span>GlobalEventHandlers</span>;
 <span>HTMLElement</span> implements <span>DocumentAndElementEventHandlers</span>;