Fix filePath being wrongly stringified (#40070)

Currently when the resolved path is `undefined` it generates `filePath: 'undefined'` in the tree object, but should be `filePath: undefined` instead.

## 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/build/webpack/loaders/next-app-loader.ts

@@ -49,7 +49,7 @@ async function createTreeCodeFromPath({
         children ? `children: ${children},` : ''
       }
     }, {
-      filePath: '${resolvedLayoutPath}',
+      filePath: ${JSON.stringify(resolvedLayoutPath)},
       ${
         resolvedLayoutPath
           ? `layout: () => require(${JSON.stringify(resolvedLayoutPath)}),`