Avoid passing null through to 'valid name prefix'

[shannonbooth]

Fixes #1387

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • Align validate and extract tests with DOM spec web-platform-tests/wpt#53533
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: bugzilla.mozilla.org/show_bug.cgi?id=1773312
  • WebKit: bugs.webkit.org/show_bug.cgi?id=241419
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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[domenic]

I guess this is worth merging as it just fixes a very clear spec bug, even if we still want to discuss further potential spec fixes in #1387, and the tests are not yet perfectly aligned with the current spec.