Ensure empty config with Turbopack does not show webpack warning

[timneutkens]

What?

There's a bug currently that shows the "webpack has been configured" warning incorrectly:

pnpm next dev --turbo examples/hello-world

> nextjs-project@0.0.0 next /Users/timneutkens/projects/next.js
> cross-env NEXT_TELEMETRY_DISABLED=1 node --trace-deprecation --enable-source-maps packages/next/dist/bin/next "dev" "--turbo" "examples/hello-world"

  ▲ Next.js 14.2.0-canary.58 (turbo)
  - Local:        http://localhost:3000

 ✓ Starting...
 ✓ Ready in 791ms
 ⚠ Webpack is configured while Turbopack is not, which may cause problems.
 ⚠ See instructions if you need to configure Turbopack:
  https://nextjs.org/docs/app/api-reference/next-config-js/turbo

This PR ensures the warning is only shown when you have actually configured webpack in next.config.js.

Closes NEXT-3013

[timneutkens]

• Ensure empty config with Turbopack does not show webpack warning #64109 👈
• canary

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[ijjk]

Failing test suites

Commit: f0b66b3

pnpm test-dev test/e2e/app-dir/app/index.test.ts (PPR)

• app dir - basic > known bugs > should support React cache > server component client-navigation

Expand output

● app dir - basic › known bugs › should support React cache › server component client-navigation

expect(received).toBe(expected) // Object.is equality

Expected: "0.7282814095165713"
Received: "0.7104008879336707"

  1424 |           const val1 = await browser.elementByCss('#value-1').text()
  1425 |           const val2 = await browser.elementByCss('#value-2').text()
> 1426 |           expect(val1).toBe(val2)
       |                        ^
  1427 |         })
  1428 |
  1429 |         it('client component', async () => {

  at Object.toBe (e2e/app-dir/app/index.test.ts:1426:24)

Read more about building and testing Next.js in contributing.md.

pnpm test-dev test/e2e/app-dir/navigation/navigation.test.ts

• app dir - navigation > browser back to a revalidated page > should load the page

Expand output

● app dir - navigation › browser back to a revalidated page › should load the page

page.waitForSelector: Timeout 60000ms exceeded.
Call log:
  - waiting for locator('#submit-button')

  421 |     return this.chain(() => {
  422 |       return page
> 423 |         .waitForSelector(selector, { timeout, state: 'attached' })
      |          ^
  424 |         .then(async (el) => {
  425 |           // it seems selenium waits longer and tests rely on this behavior
  426 |           // so we wait for the load event fire before returning

  at waitForSelector (lib/browsers/playwright.ts:423:10)

Read more about building and testing Next.js in contributing.md.