Allow the creation of null registry elements and shadow roots

Tests:
- web-platform-tests/wpt#56108
- web-platform-tests/wpt#56176

Helps with #1413.

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>

Author: ja-y-son

dom.bs

@@ -5475,7 +5475,7 @@ interface Document : Node {
 interface XMLDocument : Document {};
 
 dictionary ElementCreationOptions {
-  CustomElementRegistry customElementRegistry;
+  CustomElementRegistry? customElementRegistry;
   DOMString is;
 };
 
@@ -5847,31 +5847,36 @@ method steps are to return the result of running the
 dictionary <var>options</var> and a <a for=/>document</a> <var>document</var>:
 
 <ol>
- <li><p>Let <var>registry</var> be null.
+ <li><p>Let <var>registry</var> be the result of <a for=/>looking up a custom element registry</a>
+ given <var>document</var>.
 
  <li><p>Let <var>is</var> be null.
 
  <li>
   <p>If <var>options</var> is a dictionary:
 
   <ol>
-   <li><p>If <var>options</var>["{{ElementCreationOptions/customElementRegistry}}"]
-   <a for=map>exists</a>, then set <var>registry</var> to it.
-
-   <li><p>If <var>registry</var>'s <a for=CustomElementRegistry>is scoped</a> is false and
-   <var>registry</var> is not <var>document</var>'s <a for=Document>custom element registry</a>,
-   then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
-
    <li><p>If <var>options</var>["{{ElementCreationOptions/is}}"] <a for=map>exists</a>, then set
    <var>is</var> to it.
 
-   <li><p>If <var>registry</var> is non-null and <var>is</var> is non-null, then <a>throw</a> a
+   <li>
+    <p>If <var>options</var>["{{ElementCreationOptions/customElementRegistry}}"]
+    <a for=map>exists</a>:
+
+    <ol>
+     <li><p>If <var>is</var> is non-null, then <a>throw</a> a "{{NotSupportedError!!exception}}"
+     {{DOMException}}.
+
+     <li><p>Set <var>registry</var> to
+     <var>options</var>["{{ElementCreationOptions/customElementRegistry}}"].
+    </ol>
+
+   <li><p>If <var>registry</var> is non-null, <var>registry</var>'s
+   <a for=CustomElementRegistry>is scoped</a> is false, and <var>registry</var> is not
+   <var>document</var>'s <a for=Document>custom element registry</a>, then <a>throw</a> a
    "{{NotSupportedError!!exception}}" {{DOMException}}.
   </ol>
 
- <li><p>If <var>registry</var> is null, then set <var>registry</var> to the result of
- <a for=/>looking up a custom element registry</a> given <var>document</var>.
-
  <li><p>Return <var>registry</var> and <var>is</var>.
 </ol>
 
@@ -6672,7 +6677,7 @@ dictionary ShadowRootInit {
   SlotAssignmentMode slotAssignment = "named";
   boolean clonable = false;
   boolean serializable = false;
-  CustomElementRegistry? customElementRegistry = null;
+  CustomElementRegistry? customElementRegistry;
 };
 </pre>
 
@@ -7604,17 +7609,13 @@ are:
  <li><p>Let <var>registry</var> be <a>this</a>'s <a>node document</a>'s
  <a for=Document>custom element registry</a>.
 
- <li>
-  <p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] is non-null:
-
-  <ol>
-   <li><p>Set <var>registry</var> to <var>init</var>["{{ShadowRootInit/customElementRegistry}}"].
+ <li><p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] <a for=map>exists</a>, then
+ set <var>registry</var> to it.
 
-   <li><p>If <var>registry</var>'s <a for=CustomElementRegistry>is scoped</a> is false and
-   <var>registry</var> is not <a>this</a>'s <a>node document</a>'s
-   <a for=Document>custom element registry</a>, then <a>throw</a> a
-   "{{NotSupportedError!!exception}}" {{DOMException}}.
-  </ol>
+ <li><p>If <var>registry</var> is non-null, <var>registry</var>'s
+ <a for=CustomElementRegistry>is scoped</a> is false, and <var>registry</var> is not <a>this</a>'s
+ <a>node document</a>'s <a for=Document>custom element registry</a>, then <a>throw</a> a
+ "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li><p>Run <a>attach a shadow root</a> with <a>this</a>,
  <var>init</var>["{{ShadowRootInit/mode}}"], <var>init</var>["{{ShadowRootInit/clonable}}"],