[whatwg/webidl] Do not throw if the attribute setter is called with no arguments (PR #1498)

Do not throw if the attribute setter is called with no arguments

This is for https://github.com/whatwg/webidl/issues/1497 .

This changes the attribute setter not to throw if it's called with no arguments (this can happen only if the setter function is extracted from the property descriptor and directly called), and instead treat it as if `undefined` is passed.

No browsers had been following the previous behavior, and the updated behavior matches WebKit.

- [x] At least two implementers are interested (and none opposed):
   * WebKit (https://github.com/whatwg/webidl/issues/1497#issuecomment-3019315858)
   * Chromium (https://github.com/whatwg/webidl/issues/1497#issuecomment-3021612289, https://github.com/whatwg/webidl/issues/1497#issuecomment-3021622854)
   * Gecko (https://github.com/whatwg/webidl/issues/1497#issuecomment-3028206420 , and patch is ready in https://bugzilla.mozilla.org/show_bug.cgi?id=1974950)
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * The patch part 2 in https://bugzilla.mozilla.org/show_bug.cgi?id=1974950
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1974950
   * WebKit: (Already matches the updated behavior, and it's passing the WPT added in the above)
   * Deno: …
   * Node.js: …
   * webidl2.js: …
   * widlparser: …
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.


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

  https://github.com/whatwg/webidl/pull/1498

-- Commit Summary --

  * Do not throw if the attribute setter is called with no arguments

-- File Changes --

    M index.bs (4)

-- Patch Links --

https://github.com/whatwg/webidl/pull/1498.patch
https://github.com/whatwg/webidl/pull/1498.diff

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