fix: prerender optional parameters as empty when entries contains '*'

[LorisSigrist]

Closes #11133

Currently, any routes with parameters are only prerendered if they are discovered through crawling, or export entries.
This makes sense for required parameters, since it's not possible to know which values they accept otherwise.
For optional parameters, we do know that not passing a value is valid. With this PR routes with optional parameters that are marked as prerenderable will be crawled with no value by default.

Examples:

• /some-route/[[optionalParam]]/a will be crawled with /some-route/a
• /some-route/[[optionalParam]]/a/[[optionalParam2]]/b will be crawled with /some-route/a/b

A common situation where this causes frustration is on multi-langauge pages with a [[lang]] parameter at the root. In that case / won't be crawled. It needs to be manually specified in the entries, which feels silly.

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[benmccann]

Thanks for this!

The description of '*' should probably be tweaked to describe what happens with optional values: https://kit.svelte.dev/docs/configuration#prerender

I feel that creating a whole new test project is a bit overkill. It'd probably be better to refactor out a method that can be unit tested or add an optional route to an existing test project

[LorisSigrist]

Will do!