fix: tolerate empty posts directory with turbopackOptional

app/posts/[slug]/page.tsx

@@ -25,7 +25,10 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
 
   if (!post) notFound()
 
-  const { default: PostContent } = await import(`@/content/posts/${slug}.mdx`)
+  const { default: PostContent } = await import(
+    /* turbopackOptional: true */
+    `@/content/posts/${slug}.mdx`
+  )
 
   return (
     <>