fix: add fallback array for backwards compatibility

`preRender.notFoundRoutes` was only [introduced around v9.5.6](vercel/next.js#8832)

type PrerenderManifest = {
  version: 3
  routes: { [route: string]: SsgRoute }
  dynamicRoutes: { [route: string]: DynamicSsgRoute }
  notFoundRoutes: string[]
  preview: __ApiPreviewProps
}

packages/next-sitemap/src/url/create-url-set/index.ts

@@ -98,7 +98,7 @@ export const createUrlSet = async (
   urlSet = [...new Set(urlSet)]
 
   // Remove routes which don't exist
-  const notFoundRoutes = manifest.preRender?.notFoundRoutes as string[]
+  const notFoundRoutes = (manifest.preRender?.notFoundRoutes ?? []) as string[]
   urlSet = urlSet.filter((url) => !notFoundRoutes.includes(url))
 
   // Create sitemap fields based on transformation