Fix handling for 204 status code with a body

[ijjk]

This ensures we handle 204 and 304 status codes correctly in API routes.

Bug

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

Fixes: #28464

[styfle]

Thanks!

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
buildDuration	12.9s	12.9s	-73ms
buildDurationCached	3s	3s	-42ms
nodeModulesSize	61.7 MB	61.7 MB	⚠️ +1.52 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.311	2.304	-0.01
/ avg req/sec	1082.02	1084.96	+2.94
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.281	1.277	0
/error-in-render avg req/sec	1951.1	1958.34	+7.24

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	23.3 kB	23.3 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	67.2 kB	67.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
_app-HASH.js gzip	979 B	979 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.67 kB	2.67 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	918 B	918 B	✓
image-HASH.js gzip	4.14 kB	4.14 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	318 B	318 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	13 kB	13 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
_buildManifest.js gzip	491 B	491 B	✓
Overall change	491 B	491 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
index.html gzip	540 B	540 B	✓
link.html gzip	553 B	553 B	✓
withRouter.html gzip	534 B	534 B	✓
Overall change	1.63 kB	1.63 kB	✓

---

Webpack 4 Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
buildDuration	10.6s	10.6s	⚠️ +50ms
buildDurationCached	4.5s	4.5s	-80ms
nodeModulesSize	61.7 MB	61.7 MB	⚠️ +1.52 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.371	2.289	-0.08
/ avg req/sec	1054.32	1092.26	+37.94
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.288	1.253	-0.04
/error-in-render avg req/sec	1940.76	1995.07	+54.31

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
16.HASH.js gzip	186 B	186 B	✓
677f882d2ed8..HASH.js gzip	14.1 kB	14.1 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	10.7 kB	10.7 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	68.1 kB	68.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
_app-HASH.js gzip	964 B	964 B	✓
_error-HASH.js gzip	3.8 kB	3.8 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.87 kB	2.87 kB	✓
head-HASH.js gzip	3.06 kB	3.06 kB	✓
hooks-HASH.js gzip	924 B	924 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	3.03 kB	3.03 kB	✓
withRouter-HASH.js gzip	295 B	295 B	✓
30809af5c834..565.css gzip	125 B	125 B	✓
Overall change	18.1 kB	18.1 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
_buildManifest.js gzip	500 B	500 B	✓
Overall change	500 B	500 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/204-status	Change
index.html gzip	584 B	584 B	✓
link.html gzip	596 B	596 B	✓
withRouter.html gzip	577 B	577 B	✓
Overall change	1.76 kB	1.76 kB	✓

Commit: 8301540