Create fallback HTML file when prerendering is disabled

[dimfeld]

Fixes #4441. With prerendering.enabled = false the fallback HTML file was not created.

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Wasn't quite sure of the best way to test this. Looks like it would require creating a new sample app in the prerendering test directory with this configuration and a single test that the fallback file is written. Should I do that?

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🦋 Changeset detected

Latest commit: 1fcd33c

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[Rich-Harris]

Thank you!