This updates `fetchNextData` to re-use the `getDataHref` function from `page-loader` which has more verbose handling to ensure the correct `/_next/data` URL is built. Re-using this logic ensures the `/_next/data` URL can still be built even when a mismatching `href` and `as` value is provided to `next/link`.
This also fixes a case in `getDataHref` where optional values that weren't provided would fail to build the data href since the check requiring the param be present while interpolating the route values hasn't been updated to allow missing params for optional values.
An additional test case has been added to the prerender suite to ensure the `/_next/data` URL is built correctly when mismatching `href` and `as` values are provided
x-ref: https://github.com/vercel/next.js/discussions/14536
x-ref: https://github.com/vercel/next.js/discussions/9081#discussioncomment-31160
Closes: https://github.com/vercel/next.js/issues/14668
Since non-fallback pages don't rely on the URL for hydration we can allow them to be rewritten to but pages with fallback still can't be rewritten to because we won't be able to parse the correct `/_next/data` path to request the page's data from. I added a test for this behavior and ensured it works correctly on Now.
Example on with fallback false rewrite on Now:
https://tst-rewrite-cp9vge4bg.now.sh/about
