Ensure error is shown correctly for empty headers field (#28430)

* Ensure error is shown correctly for empty headers field

* Apply suggestions from code review

Co-authored-by: Steven <steven@ceriously.com>

Co-authored-by: Steven <steven@ceriously.com>

Author: ijjk

packages/next/lib/load-custom-routes.ts

@@ -108,6 +108,8 @@ function checkHeader(route: Header): string[] {
 
   if (!Array.isArray(route.headers)) {
     invalidParts.push('`headers` field must be an array')
+  } else if (route.headers.length === 0) {
+    invalidParts.push('`headers` field cannot be empty')
   } else {
     for (const header of route.headers) {
       if (!header || typeof header !== 'object') {

test/integration/invalid-custom-routes/test/index.test.js

@@ -25,6 +25,23 @@ const writeConfig = async (routes, type = 'redirects') => {
 let getStderr
 
 const runTests = () => {
+  it('should error when empty headers array is present on header item', async () => {
+    await writeConfig(
+      [
+        {
+          source: `/:path*`,
+          headers: [],
+        },
+      ],
+      'headers'
+    )
+    const stderr = await getStderr()
+
+    expect(stderr).toContain(
+      '`headers` field cannot be empty for route {"source":"/:path*"'
+    )
+  })
+
   it('should error when source and destination length is exceeded', async () => {
     await writeConfig(
       [