[w3c/ServiceWorker] Use `undefined` union type for getRegistration() (#1548)

This is now possible thanks to https://github.com/heycam/webidl/pull/906.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1548

-- Commit Summary --

  * Use `undefined` union type for getRegistration()

-- File Changes --

    M docs/index.bs (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1548.patch
https://github.com/w3c/ServiceWorker/pull/1548.diff

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https://github.com/w3c/ServiceWorker/pull/1548