Always enqueue an element regardless of callback

Tests: ...

Fixes WICG/webcomponents#760.

source

@@ -67033,24 +67033,28 @@ customElements.define("x-foo", class extends HTMLElement {
    data-x="concept-custom-element-definition-lifecycle-callbacks">lifecycle callbacks</span> with
    key <var>callbackName</var>.</p></li>
 
-   <li><p>If <var>callback</var> is null, then return</p></li>
-
    <li>
-    <p>If <var>callbackName</var> is "<code data-x="">attributeChangedCallback</code>", then:</p>
+    <p>If <var>callback</var> is non-null, then:</p>
 
     <ol>
-     <li><p>Let <var>attributeName</var> be the first element of <var>args</var>.</p></li>
+     <li>
+      <p>If <var>callbackName</var> is "<code data-x="">attributeChangedCallback</code>", then:</p>
+
+      <ol>
+       <li><p>Let <var>attributeName</var> be the first element of <var>args</var>.</p></li>
 
-     <li><p>If <var>definition</var>'s <span
-     data-x="concept-custom-element-definition-observed-attributes">observed attributes</span> does
-     not contain <var>attributeName</var>, then return.</p></li>
+       <li><p>If <var>definition</var>'s <span
+       data-x="concept-custom-element-definition-observed-attributes">observed attributes</span>
+       does not contain <var>attributeName</var>, then return.</p></li>
+      </ol>
+     </li>
+
+     <li><p>Add a new <span>callback reaction</span> to <var>element</var>'s <span>custom element
+     reaction queue</span>, with callback function <var>callback</var> and arguments
+     <var>args</var>.</p></li>
     </ol>
    </li>
 
-   <li><p>Add a new <span>callback reaction</span> to <var>element</var>'s <span>custom element
-   reaction queue</span>, with callback function <var>callback</var> and arguments
-   <var>args</var>.</p></li>
-
    <li><p><span>Enqueue an element on the appropriate element queue</span> given
    <var>element</var>.</p></li>
   </ol>