Fix ISG mode (revalidate: true) currently broken (redirects to 404)

**Describe the bug**
A recent change in the handling of 404 pages broke the ISG feature. Instead of generating the page at runtime, it nows redirects to a 404 page.

https://nrn-v2-mst-aptd-gcms-lcz-sty-c1.vercel.app/fr/examples/native-features/example-with-ssg-and-revalidate

