[css-transitions] Specify that a 'none' shadow list is like a zero-length list.

This makes interpolations between inset shadows and 'none' work
correctly, fixing the issue reported in
https://lists.w3.org/Archives/Public/www-style/2013Feb/0139.html

Author: dbaron

css-transitions/Overview.bs

@@ -1336,8 +1336,9 @@ url: https://html.spec.whatwg.org/multipage/infrastructure.html#concept-event-di
           endpoint; if neither value is ''visibility/visible'' then not interpolable.
         </li>
         <li id="animtype-shadow-list">
-          <strong>shadow list</strong>: Each shadow in the list is
-          interpolated via the
+          <strong>shadow list</strong>: Each shadow in the list
+          (treating ''shadow/none'' as a 0-length list)
+          is interpolated via the
           color (as <a href="#animtype-color">color</a>) component,
           and x, y, blur, and (when appropriate) spread
           (as <a href="#animtype-length">length</a>) components.

css-transitions/Overview.html

@@ -105,7 +105,7 @@
    <h1 class="p-name no-ref" id="title">CSS Transitions</h1>
 
    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft,
-    <time class="dt-updated" datetime="2015-06-09">9 June 2015</time></span></h2>
+    <time class="dt-updated" datetime="2015-08-12">12 August 2015</time></span></h2>
 
    <div data-fill-with="spec-metadata">
     <dl>
@@ -1963,8 +1963,9 @@ <h3 class="heading settled" data-level="7.1" id="interpolated-types"><span class
 
 
     <li id="animtype-shadow-list"><a class="self-link" href="#animtype-shadow-list"></a>
-          <strong>shadow list</strong>: Each shadow in the list is
-          interpolated via the
+          <strong>shadow list</strong>: Each shadow in the list
+          (treating <span class="css">none</span> as a 0-length list)
+          is interpolated via the
           color (as <a href="#animtype-color">color</a>) component,
           and x, y, blur, and (when appropriate) spread
           (as <a href="#animtype-length">length</a>) components.