fix: should allow nullable param for res.end (#28694)

## Bug
Fixes #28693

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/api-utils.ts

@@ -69,12 +69,12 @@ export async function apiResolver(
     const writeData = apiRes.write
     const endResponse = apiRes.end
     apiRes.write = (...args: any[2]) => {
-      contentLength += Buffer.byteLength(args[0])
+      contentLength += Buffer.byteLength(args[0] || '')
       return writeData.apply(apiRes, args)
     }
     apiRes.end = (...args: any[2]) => {
       if (args.length && typeof args[0] !== 'function') {
-        contentLength += Buffer.byteLength(args[0])
+        contentLength += Buffer.byteLength(args[0] || '')
       }
 
       if (contentLength >= 4 * 1024 * 1024) {

test/integration/api-support/pages/api/nullable-payload.js

@@ -0,0 +1,4 @@
+export default (req, res) => {
+  res.write('')
+  res.end()
+}

test/integration/api-support/test/index.test.js

@@ -442,6 +442,11 @@ function runTests(dev = false) {
     expect(data).toBe('hi')
   })
 
+  it('should work with nullable payload', async () => {
+    const data = await renderViaHTTP(appPort, '/api/nullable-payload')
+    expect(data).toBe('')
+  })
+
   it('should warn if response body is larger than 4MB', async () => {
     let res = await fetchViaHTTP(appPort, '/api/large-response')
     expect(res.ok).toBeTruthy()