Ensure config's experimental field exists

[bennettdams]

Fixes #34499

Starting with v12.1.0, you can't use React 18 when you don't use the experimental field in the next.config.js

That's because this recent change sets reactRoot on the user's config without checking if the key already exists:

next.js/packages/next/server/config.ts

Lines 679 to 682 in 787186a

	const hasReactRoot = shouldUseReactRoot()
	if (hasReactRoot) {
	userConfig.experimental.reactRoot = true
	}

This change initializes experimental on the userConfig if necessary.

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	bennettdams/next.js patch-2	Change
buildDuration	15.3s	15.3s	-49ms
buildDurationCached	6.1s	6.1s	-29ms
nodeModulesSize	359 MB	359 MB	⚠️ +251 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	bennettdams/next.js patch-2	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.005	3.023	⚠️ +0.02
/ avg req/sec	832.02	827.12	⚠️ -4.9
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.182	1.171	-0.01
/error-in-render avg req/sec	2114.82	2134.88	+20.06

Client Bundles (main, webpack, commons)

	vercel/next.js canary	bennettdams/next.js patch-2	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	27.9 kB	27.9 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.5 kB	71.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	bennettdams/next.js patch-2	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	bennettdams/next.js patch-2	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	326 B	326 B	✓
dynamic-HASH.js gzip	2.57 kB	2.57 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	919 B	919 B	✓
image-HASH.js gzip	5.05 kB	5.05 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.26 kB	2.26 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.7 kB	14.7 kB	✓

Client Build Manifests

	vercel/next.js canary	bennettdams/next.js patch-2	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	bennettdams/next.js patch-2	Change
index.html gzip	532 B	532 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	bennettdams/next.js patch-2	Change
buildDuration	18.7s	18.7s	⚠️ +38ms
buildDurationCached	6s	6.1s	⚠️ +57ms
nodeModulesSize	359 MB	359 MB	⚠️ +251 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	bennettdams/next.js patch-2	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.003	2.99	-0.01
/ avg req/sec	832.45	836.18	+3.73
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.167	1.183	⚠️ +0.02
/error-in-render avg req/sec	2142.05	2113.5	⚠️ -28.55

Client Bundles (main, webpack, commons)

	vercel/next.js canary	bennettdams/next.js patch-2	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.1 kB	42.1 kB	✓
main-HASH.js gzip	27.9 kB	27.9 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.7 kB	71.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	bennettdams/next.js patch-2	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	bennettdams/next.js patch-2	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.56 kB	2.56 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	911 B	911 B	✓
image-HASH.js gzip	5.08 kB	5.08 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.28 kB	2.28 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.7 kB	14.7 kB	✓

Client Build Manifests

	vercel/next.js canary	bennettdams/next.js patch-2	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	bennettdams/next.js patch-2	Change
index.html gzip	533 B	533 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 3dbf718