[fix] fallback should still be generated when prerender is disabled

[Theo-Steiner]

Fixes #1588

The bug described in #1588 is about the adapter-static in SPA-mode (-> fallback set) not outputting the {fallback}.html file into the build directory when config.kit.prerender.enabled is set to false.

This PR is an easy fix for this behavior, as config.kit.prerender.enabled can be ignored in the SPA mode. It bypasses the check for config.kit.prerender.enabled in the getter for the prerender function by modifying the expression to be true by default when a fallback is defined.

You can see that this fixes the bug on the branch bug-demonstration on my fork of sveltejs/kit.

For this,

1. clone the repo git clone https://github.com/Theo-Steiner/kit.git
2. run checkout bug-demonstration, pnpm i, pnpm build
3. navigate to cd examples/kit-bug
4. execute pnpm build and do ls build and you will see that index.html was in fact created!

• Ideally, include a test that fails without this PR but passes with it.

I did not include a test, since I honestly don't know how I would test for this behavior.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 598748a

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Theo-Steiner]

@benmccann I made the crawling for loop conditional as you suggested! Thank's for the input =)

[benmccann]

@Theo-Steiner lgtm, but the tests are failing and will need to be updated. I added a changeset, so you might want to do a git pull first to pull that change down locally before updating the code