Correctly set "is value" in [HTMLConstructor]

Fixes #3776.

Tests: already present in
web-platform-tests/wpt@1e4fe87#diff-d2e09155cd799584f19630072186bff5,
as we forgot to update this part of the HTML Standard when we merged
whatwg/dom#566.

Author: domenic

source

@@ -10286,6 +10286,8 @@ document.createElement("bad-1");  // (2)</pre>
     NewTarget will be undefined).</p>
    </li>
 
+   <li><p>Let <var>is value</var> be null.</p></li>
+
    <li>
     <p>If <var>definition</var>'s <span data-x="concept-custom-element-definition-local-name">local
     name</span> is equal to <var>definition</var>'s <span
@@ -10338,6 +10340,9 @@ document.createElement("bad-1");  // (2)</pre>
        which is not in that list.</p>
       </div>
      </li>
+
+     <li><p>Set <var>is value</var> to <var>definition</var>'s <span
+     data-x="concept-custom-element-definition-name">name</span>.</p></li>
     </ol>
    </li>
 
@@ -10385,6 +10390,9 @@ document.createElement("bad-1");  // (2)</pre>
      <li><p>Set <var>element</var>'s <span data-x="concept-element-custom-element-definition">custom
      element definition</span> to <var>definition</var>.</p></li>
 
+     <li><p>Set <var>element</var>'s <span data-x="concept-element-is-value"><code
+     data-x="">is</code> value</span> to <var>is value</var>.</p></li>
+
      <li><p>Return <var>element</var>.</p></li>
     </ol>