[cssom] Make MediaList#mediaText have [TreatNullAs=EmptyString]. https://www.w3.org/Bugs/Public/show_bug.cgi?id=22308

Author: Simon Pieters

cssom/Overview.html

@@ -590,7 +590,7 @@ <h3 id="the-medialist-interface"><span class="secno">4.4 </span>The <code title=
 
 <pre class="idl">[<a href="#dom-medialist" title="dom-MediaList">Constructor</a>(DOMString text)]
 interface <dfn id="medialist">MediaList</dfn> {
-  stringifier attribute DOMString <a href="#dom-medialist-mediatext" title="dom-MediaList-mediaText">mediaText</a>;
+  [TreatNullAs=EmptyString] stringifier attribute DOMString <a href="#dom-medialist-mediatext" title="dom-MediaList-mediaText">mediaText</a>;
   readonly attribute unsigned long <a href="#dom-medialist-length" title="dom-MediaList-length">length</a>;
   getter DOMString <a href="#dom-medialist-item" title="dom-MediaList-item">item</a>(unsigned long index);
   void <a href="#dom-medialist-appendmedium" title="dom-MediaList-appendMedium">appendMedium</a>(DOMString medium);

cssom/Overview.src.html

@@ -513,7 +513,7 @@ <h3>The <code title>MediaList</code> Interface</h3>
 
 <pre class=idl>[<span title=dom-MediaList>Constructor</span>(DOMString text)]
 interface <dfn>MediaList</dfn> {
-  stringifier attribute DOMString <span title=dom-MediaList-mediaText>mediaText</span>;
+  [TreatNullAs=EmptyString] stringifier attribute DOMString <span title=dom-MediaList-mediaText>mediaText</span>;
   readonly attribute unsigned long <span title=dom-MediaList-length>length</span>;
   getter DOMString <span title=dom-MediaList-item>item</span>(unsigned long index);
   void <span title=dom-MediaList-appendMedium>appendMedium</span>(DOMString medium);