fix: nav jumping around if no nav preferences are set (#9301)

If you start a fresh dev server and open payload, the nav will initially
show as closed and then jump to its open state. This is because no
preferences are set, so the server tells the client to initially keep it
closed, despite the default nav state being _open_.

packages/next/src/layouts/Root/index.tsx

@@ -102,7 +102,7 @@ export const RootLayout = async ({
           config={clientConfig}
           dateFNSKey={i18n.dateFNSKey}
           fallbackLang={config.i18n.fallbackLanguage}
-          isNavOpen={navPrefs?.open}
+          isNavOpen={navPrefs?.open ?? true}
           languageCode={languageCode}
           languageOptions={languageOptions}
           permissions={permissions}