Throw for new FormData() when "constructing the entry list" returns null

Tests: web-platform-tests/wpt#14637.

Change to HTML: whatwg/html#4239.

xhr.bs

@@ -1776,8 +1776,18 @@ constructor must run these steps:
 <ol>
  <li><p>Let <var>fd</var> be a new {{FormData}} object.
 
- <li><p>If <var>form</var> is given, then set <var>fd</var>'s <a for=FormData>entry list</a> to the
- result of <a>constructing the entry list</a> for <var>form</var>.
+ <li>
+  <p>If <var>form</var> is given, then:
+
+  <ol>
+   <li><p>Let <var>list</var> be the result of <a>constructing the entry list</a> for
+   <var>form</var>.
+
+   <li><p>If <var>list</var> is null, then <a>throw</a> an "{{InvalidStateError!!exception}}"
+   {{DOMException}}.
+
+   <li><p>Set <var>fd</var>'s <a for=FormData>entry list</a> to <var>list</var>.
+  </ol>
 
  <li><p>Return <var>fd</var>.
 </ol>