This fixes revalidation not occurring correctly when `notFound: true` is returned during build, additional tests have been added to ensure this is working correctly for dynamic and non-dynamic pages returning `notFound: true` during build and then revalidating afterwards.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`
Fixes: https://github.com/vercel/next.js/issues/21453