Fix attempting to get the chunk size after strategy is cleared

[domenic]

Although a better fix might be to delay size calculation until we've verified that we're not in the erroring or errored states, that has observable differences for certain bad-strategy cases already in the WPT suite, and multiple implementations seem to have converged on this particular fix already.

Closes #1331.

• At least two implementers are interested (and none opposed):
  • Already implemented in Chrome
  • Already implemented in Firefox
  • Other implementations behave observably the same way as those
• Tests are written and can be reviewed and commented upon at:
  • Already tested
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: N/A
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=283739
  • Deno: Streams: writableStreamDefaultControllerGetChunkSize undefined strategy case slightly broken denoland/deno#27102
  • Node.js: Streams: writableStreamDefaultControllerGetChunkSize undefined strategy case slightly broken nodejs/node#56014
• MDN issue is filed: small edge case; N/A
• The top of this comment includes a clear commit message to use.

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