fix: handle notFound: true in / with next export (#40592)

Closes #40571

An earlier fix in #24481 did not consider the `/` case. The page path normalization method `normalizePagePath` turned `/` into `/index` and the route matching was skipped for the index route's non-existent HTML file.

## Bug

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

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

packages/next/export/index.ts

@@ -640,13 +640,13 @@ export default async function exportApp(
         Object.keys(prerenderManifest.routes).map(async (route) => {
           const { srcRoute } = prerenderManifest!.routes[route]
           const pageName = srcRoute || route
-          route = normalizePagePath(route)
 
           // returning notFound: true from getStaticProps will not
           // output html/json files during the build
           if (prerenderManifest!.notFoundRoutes.includes(route)) {
             return
           }
+          route = normalizePagePath(route)
 
           const pagePath = getPagePath(pageName, distDir, isLikeServerless)
           const distPagesDir = join(

test/integration/export-index-not-found-gsp/pages/index.js

@@ -0,0 +1,7 @@
+export default function Page() {
+  return 'Hello world'
+}
+
+export function getStaticProps() {
+  return { notFound: true }
+}

test/integration/export-index-not-found-gsp/test/index.test.ts

@@ -0,0 +1,21 @@
+/* eslint-env jest */
+
+import fs from 'fs-extra'
+import { join } from 'path'
+import { nextBuild, nextExport } from 'next-test-utils'
+
+const appDir = join(__dirname, '../')
+const outdir = join(appDir, 'out')
+
+describe('Export index page with `notFound: true` in `getStaticProps`', () => {
+  it('should build successfully', async () => {
+    await fs.remove(join(appDir, '.next'))
+    const { code } = await nextBuild(appDir)
+    if (code !== 0) throw new Error(`build failed with status ${code}`)
+  })
+
+  it('should export successfully', async () => {
+    const { code } = await nextExport(appDir, { outdir })
+    if (code !== 0) throw new Error(`export failed with status ${code}`)
+  })
+})