Ensure 404 with SSG is rendered correctly with notFound

[ijjk]

This ensures a custom /404 page with getStaticProps works correctly when leveraging the new unstable_notFound support in getStaticProps

Closes: #18196
x-ref: #17755

[ijjk]

Stats from current PR

Default Server Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
buildDuration	13.5s	13.5s	⚠️ +35ms
nodeModulesSize	91.3 MB	91.3 MB	⚠️ +938 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.522	2.453	-0.07
/ avg req/sec	991.36	1019.35	+27.99
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.435	1.391	-0.04
/error-in-render avg req/sec	1742.35	1796.93	+54.58

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
677f882d2ed8..574c.js gzip	11.2 kB	11.2 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-af76201..828d.js gzip	7.34 kB	7.34 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.3 kB	58.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
677f882d2ed8..dule.js gzip	7.02 kB	7.02 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-3ba933e..dule.js gzip	6.32 kB	6.32 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.1 kB	53.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-41ceea0..6b62.js gzip	1.35 kB	1.35 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.75 kB	7.75 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-2d41a89..dule.js gzip	1.31 kB	1.31 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.4 kB	5.4 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
index.html gzip	1 kB	1 kB	✓
link.html gzip	1.01 kB	1.01 kB	✓
withRouter.html gzip	995 B	995 B	✓
Overall change	3 kB	3 kB	✓

---

Serverless Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
buildDuration	15.8s	15.1s	-666ms
nodeModulesSize	91.3 MB	91.3 MB	⚠️ +938 B

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
677f882d2ed8..574c.js gzip	11.2 kB	11.2 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-af76201..828d.js gzip	7.34 kB	7.34 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.3 kB	58.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
677f882d2ed8..dule.js gzip	7.02 kB	7.02 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-3ba933e..dule.js gzip	6.32 kB	6.32 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.1 kB	53.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-41ceea0..6b62.js gzip	1.35 kB	1.35 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.75 kB	7.75 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-2d41a89..dule.js gzip	1.31 kB	1.31 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.4 kB	5.4 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Serverless bundles Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js ssg/404-not-found	Change
_error.js	1.06 MB	1.06 MB	⚠️ +140 B
404.html	4.73 kB	4.73 kB	✓
hooks.html	3.92 kB	3.92 kB	✓
index.js	1.06 MB	1.06 MB	⚠️ +140 B
link.js	1.1 MB	1.1 MB	⚠️ +140 B
routerDirect.js	1.1 MB	1.1 MB	⚠️ +140 B
withRouter.js	1.1 MB	1.1 MB	⚠️ +140 B
Overall change	5.42 MB	5.42 MB	⚠️ +700 B

Commit: 6e69317