examples(with-turbopack): Fix styling page default active stylingNav (vercel#42739)

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Closes: vercel#42768

## Bug

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

## Documentation / Examples

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

Co-authored-by: JJ Kasper <[email protected]>

Author: m7yue

examples/with-turbopack/app/styling/StylingNav.tsx

@@ -31,14 +31,14 @@ const items = [
 ];
 
 const StylingNav = () => {
-  const [selectedLayoutSegments] = useSelectedLayoutSegments();
+  const [selectedLayoutSegments = ''] = useSelectedLayoutSegments();
 
   return (
     <div className="flex items-center space-x-4">
       {items.map((item) => (
         <TabNavItem
           key={item.slug}
-          href={`/styling/${item.slug}`}
+          href={`/styling${item.slug ? `/${item.slug}` : ''}`}
           isActive={item.slug === selectedLayoutSegments}
         >
           {item.name}