Allow passing null for customElementRegistry

Apparently people are passing a ShadowRoot instance as the argument to attachShadow().

Context: https://bugs.webkit.org/show_bug.cgi?id=295174. We'll add tests as part of fixing this in WebKit.

Author: annevk

dom.bs

@@ -6598,7 +6598,7 @@ dictionary ShadowRootInit {
   SlotAssignmentMode slotAssignment = "named";
   boolean clonable = false;
   boolean serializable = false;
-  CustomElementRegistry customElementRegistry;
+  CustomElementRegistry customElementRegistry? = null;
 };
 </pre>
 
@@ -7509,8 +7509,8 @@ are:
 <ol>
  <li><p>Let <var>registry</var> be <a>this</a>'s <a for=Element>custom element registry</a>.
 
- <li><p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] <a for=map>exists</a>, then
- set <var>registry</var> to it.
+ <li><p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] is non-null, then set
+ <var>registry</var> to it.
 
  <li><p>Run <a>attach a shadow root</a> with <a>this</a>,
  <var>init</var>["{{ShadowRootInit/mode}}"], <var>init</var>["{{ShadowRootInit/clonable}}"],