[whatwg/dom] Allow the creation of null registry element and shadow root (PR #1424)

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As part of the effort to support null custom element registry elements without shadow DOM, we're updating the spec to allow the creation of element and shadowroot to use null as a valid argument and create null registry element and shadowroot.

Fixes #1413 partially. We need another PR to support element attribute for null custom element registry in addition to #1423.


- [x] At least two implementers are interested (and none opposed):
   * webkit
   * chromium
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * … <!-- If these tests are tentative, link a PR to make them non-tentative. -->
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: …
   * Deno (only for aborting and events): …
   * Node.js (only for aborting and events): …
- [x] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: N/A
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1424

-- Commit Summary --

  * Allow the creation of null registry element and shadowroot

-- File Changes --

    M dom.bs (32)

-- Patch Links --

https://github.com/whatwg/dom/pull/1424.patch
https://github.com/whatwg/dom/pull/1424.diff

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