Omit service-worker-index.html when service-worker.js does not exist

[leaysgur]

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Tests

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The service-worker-index.html should be generated only when we use ServiceWorker.

[benmccann]

this looks good to me with just one minor comment

[leaysgur]

@benmccann
Thank you for the review 😉
I deleted that comment.

[benmccann]

lgtm

[benmccann]

@leader22 would you be able to rebase this PR?

[leaysgur]

@benmccann I resolved conflicts but one test(unrelated to this PR) failed.

135 passing (1m)
1 failing

1) export-webpack
     "before all" hook: build app for "injects <link rel=preload> tags":
   Error: Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/Users/runner/work/sapper/sapper/test/apps/export-webpack/test.ts)

Can you re-run that action?