fix(next): prevent locale upsert when not authenticated (#13621)

### What?

When `?locale=` is present in an admin panel URL and that admin panel
URL is visited in an unauthenticated browser, a runtime error is thrown.

### Why?

`upsertPreferences` relies on `req.user` to successfully create a new
`payload-preferences` document. When an unauthenticated user visits a
URL with a `locale` parameter, `upsertPreferences` is called but
`req.user` is not available.

### How?

Prevent `upsertPreferences` from being called when `req.user` is not
available.

Fixes #13581

Co-authored-by: Patrik Kozak <35232443+PatrikKozak@users.noreply.github.com>

packages/next/src/utilities/getRequestLocale.ts

@@ -13,7 +13,7 @@ export async function getRequestLocale({ req }: GetRequestLocalesArgs): Promise<
   if (req.payload.config.localization) {
     const localeFromParams = req.query.locale as string | undefined
 
-    if (localeFromParams) {
+    if (req.user && localeFromParams) {
       await upsertPreferences<Locale['code']>({ key: 'locale', req, value: localeFromParams })
     }