Remove empty config warning (#49435)

## What?

Removes the empty `next.config.js` warning. Given that `create-next-app` now ships with an empty config with the type definition comment the warning is no longer needed.

## How?

Removed the warning, removed the tests.

Fixes NEXT-1107

packages/next/src/server/config.ts

@@ -779,12 +779,6 @@ export default async function loadConfig(
       }
     }
 
-    if (Object.keys(userConfig).length === 0) {
-      curLog.warn(
-        `Detected ${configFileName}, no exported configuration found. https://nextjs.org/docs/messages/empty-configuration`
-      )
-    }
-
     if (userConfig.target && userConfig.target !== 'server') {
       throw new Error(
         `The "target" property is no longer supported in ${configFileName}.\n` +

test/integration/config-empty/next.config.js

@@ -1,6 +0,0 @@
-/* eslint-disable */
-{
-  experimental: {
-    basePath: '/docs'
-  }
-}

test/integration/config-empty/pages/index.js

@@ -1 +0,0 @@
-export default () => <div>Hello World</div>

test/integration/config-empty/test/index.test.js

@@ -1,42 +0,0 @@
-/* eslint-env jest */
-
-import { join } from 'path'
-import {
-  nextBuild,
-  launchApp,
-  findPort,
-  killApp,
-  waitFor,
-} from 'next-test-utils'
-
-const appDir = join(__dirname, '..')
-
-describe('Empty configuration', () => {
-  it('should show relevant warning and compile successfully for next build', async () => {
-    const { stderr, stdout } = await nextBuild(appDir, [], {
-      stderr: true,
-      stdout: true,
-    })
-    expect(stdout).toMatch(/Compiled successfully/)
-    expect(stderr).toMatch(
-      /Detected next\.config\.js, no exported configuration found\. https:\/\//
-    )
-  })
-
-  it('should show relevant warning and compile successfully for next dev', async () => {
-    let stderr = ''
-
-    const appPort = await findPort()
-    const app = await launchApp(appDir, appPort, {
-      onStderr(msg) {
-        stderr += msg || ''
-      },
-    })
-    await waitFor(1000)
-    await killApp(app)
-
-    expect(stderr).toMatch(
-      /Detected next\.config\.js, no exported configuration found\. https:\/\//
-    )
-  })
-})