Use default next.config.js if not found

[styfle]

Some warnings/errors will mention next.config.js even if it doesn't exist so we'll make sure to assign a default value of next.config.js.

• Follow up to Ensure error message prints next.config.mjs #30152

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
buildDuration	15.5s	15.6s	⚠️ +104ms
buildDurationCached	3.5s	3.5s	-17ms
nodeModulesSize	196 MB	196 MB	⚠️ +85 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.544	3.412	-0.13
/ avg req/sec	705.49	732.7	+27.21
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.767	1.667	-0.1
/error-in-render avg req/sec	1414.63	1499.55	+84.92

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
651.cd440d20..0b23.js gzip	2.96 kB	2.96 kB	✓
831.695e33f6..205f.js gzip	179 B	179 B	✓
framework-89..a097.js gzip	42.2 kB	42.2 kB	✓
main-2d2c447..b00a.js gzip	26.1 kB	26.1 kB	✓
webpack-f09b..493e.js gzip	1.47 kB	1.47 kB	✓
Overall change	73 kB	73 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
_app-9fb8765..c13d.js gzip	979 B	979 B	✓
_error-d828d..2954.js gzip	3.06 kB	3.06 kB	✓
amp-5388d1f5..4ce9.js gzip	551 B	551 B	✓
css-10424225..367a.js gzip	329 B	329 B	✓
dynamic-9821..3cd4.js gzip	2.67 kB	2.67 kB	✓
head-1df323b..d261.js gzip	2.32 kB	2.32 kB	✓
hooks-ab0016..b093.js gzip	917 B	917 B	✓
image-c3cc30..4e2a.js gzip	5.87 kB	5.87 kB	✓
index-95c8cb..6970.js gzip	260 B	260 B	✓
link-05c99e7..35ec.js gzip	1.66 kB	1.66 kB	✓
routerDirect..6659.js gzip	319 B	319 B	✓
script-d94ba..ed05.js gzip	386 B	386 B	✓
withRouter-7..8b2e.js gzip	317 B	317 B	✓
9a34b27eb3f9..27d.css gzip	125 B	125 B	✓
Overall change	19.8 kB	19.8 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
_buildManifest.js gzip	458 B	458 B	✓
Overall change	458 B	458 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
index.html gzip	536 B	536 B	✓
link.html gzip	548 B	548 B	✓
withRouter.html gzip	529 B	529 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
buildDuration	7.7s	7.7s	⚠️ +49ms
buildDurationCached	3.5s	3.6s	⚠️ +143ms
nodeModulesSize	196 MB	196 MB	⚠️ +85 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.482	3.585	⚠️ +0.1
/ avg req/sec	717.88	697.31	⚠️ -20.57
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.783	1.767	-0.02
/error-in-render avg req/sec	1402.27	1415.11	+12.84

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
450.fd3ad245..022b.js gzip	179 B	179 B	✓
675-228d3e2f..0c27.js gzip	13.8 kB	13.8 kB	✓
framework-13..70b0.js gzip	50.8 kB	50.8 kB	✓
main-3b9ef1a..94c9.js gzip	36.5 kB	36.5 kB	✓
webpack-a7f2..2650.js gzip	1.63 kB	1.63 kB	✓
Overall change	103 kB	103 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
_app-ae91bc3..d985.js gzip	1.33 kB	1.33 kB	✓
_error-273ac..9ef0.js gzip	180 B	180 B	✓
amp-5f9a7694..99ce.js gzip	315 B	315 B	✓
css-3a1b2477..df1e.js gzip	330 B	330 B	✓
dynamic-e04a..a1b5.js gzip	2.79 kB	2.79 kB	✓
head-7f600d0..daa3.js gzip	356 B	356 B	✓
hooks-bd8c02..0730.js gzip	638 B	638 B	✓
image-16df09..4584.js gzip	555 B	555 B	✓
index-80be94..e89f.js gzip	262 B	262 B	✓
link-4ee1ea7..8745.js gzip	2.22 kB	2.22 kB	✓
routerDirect..c4aa.js gzip	325 B	325 B	✓
script-67da0..49e6.js gzip	390 B	390 B	✓
withRouter-2..2409.js gzip	322 B	322 B	✓
9a34b27eb3f9..27d.css gzip	125 B	125 B	✓
Overall change	10.1 kB	10.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
_buildManifest.js gzip	477 B	477 B	✓
Overall change	477 B	477 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js use-default-next-config-js-if-not-found	Change
index.html gzip	535 B	535 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 82287b1