Crawler doesn't discover / if there is an optional parameter at the base

[LorisSigrist]

Describe the bug

The Crawler for prerendering does not discover the / path, if there is an optional Parameter at the base.
This is a common scenario in multi-language pages.

routes
|- layout.js //enable prerender
|- [[lang]]
    |- +page.svelte //Not found

I would expect / to get crawled, and further pages to be discovered from there.
The workaround is to add / to the entries in svelte.config.js

Reproduction

https://github.com./LorisSigrist/sk-crawler-optional-param-at-base-reproduction

Logs

Error: The following routes were marked as prerenderable, but were not prerendered because they were not found while crawling your app:
  - /[[lang]]

System Info

System:
    OS: macOS 14.0
    CPU: (8) arm64 Apple M1
    Memory: 101.14 MB / 8.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 21.2.0 - /opt/homebrew/bin/node
    Yarn: 1.22.19 - /opt/homebrew/bin/yarn
    npm: 10.2.3 - /opt/homebrew/bin/npm
    pnpm: 8.11.0 - /opt/homebrew/bin/pnpm
    bun: 0.8.1 - /opt/homebrew/bin/bun
  Browsers:
    Chrome: 119.0.6045.159
    Chrome Canary: 121.0.6153.0
    Safari: 17.0
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.1.1 
    @sveltejs/kit: ^1.27.4 => 1.27.6 
    svelte: ^4.2.7 => 4.2.7 
    vite: ^4.4.2 => 4.5.0

Severity

annoyance

Additional Information

No response