Fix middleware 404 prefetch case (#46782)

Ensures we properly generate the 404 page when a path is being prefetched with middleware configured. 

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

Closes: https://github.com/vercel/next.js/issues/38239#issuecomment-1454928413

packages/next/src/server/base-server.ts

@@ -1196,7 +1196,11 @@ export default abstract class Server<ServerOptions extends Options = Options> {
     // when we are handling a middleware prefetch and it doesn't
     // resolve to a static data route we bail early to avoid
     // unexpected SSR invocations
-    if (!isSSG && req.headers['x-middleware-prefetch']) {
+    if (
+      !isSSG &&
+      req.headers['x-middleware-prefetch'] &&
+      !(is404Page || pathname === '/_error')
+    ) {
       res.setHeader('x-middleware-skip', '1')
       res.body('{}').send()
       return null

test/e2e/middleware-general/test/index.test.ts

@@ -80,6 +80,12 @@ describe('Middleware Runtime', () => {
         /This page could not be found/
       )
       expect(await browser.eval('window.beforeNav')).toBe(1)
+
+      await browser.refresh()
+      await check(
+        () => browser.eval('document.documentElement.innerHTML'),
+        /This page could not be found/
+      )
     })
 
     it('should be able to rewrite on _next/static/chunks/pages/ 404', async () => {