[whatwg/fetch] Update fetch-later spec according to WPT expectations=0D (PR #1902)

There were two missing parts:

1. The body can either be null or empty (it was missing the
empty case)
2. Quota computation shouldn't include aborted or already
completed requests. WPT uses `AbortController` to ensure
quota is freed up. This also makes sense, since they are
no longer relevant. Therefore, only consider pending
deferred fetches when computing quota.

- [ ] At least two implementers are interested (and none opposed):
   * …
   * …
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://wpt.fyi/results/fetch/fetch-later/quota/empty-payload.https.window.html?label=experimental&label=master&aligned
   * https://wpt.fyi/results/fetch/fetch-later/quota/accumulated-oversized-payload.https.window.html?label=experimental&label=master&aligned
- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: n/a already passing tests
   * Gecko: n/a no implementation yet
   * WebKit: n/a no implementation yet
- [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.)


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

  https://github.com/whatwg/fetch/pull/1902

-- Commit Summary --

  * Update fetch-later spec according to WPT expectations

-- File Changes --

    M fetch.bs (5)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1902.patch
https://github.com/whatwg/fetch/pull/1902.diff

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