fix(posts): guard generateStaticParams against empty content dir

app/posts/[slug]/page.tsx

@@ -10,6 +10,9 @@ export const dynamic = 'force-static'
 
 export async function generateStaticParams() {
   const posts = await getPosts()
+  if (posts.length === 0) {
+    return [{ slug: '__no-posts__' }]
+  }
   return posts.map((post) => ({ slug: post.slug }))
 }
 
@@ -22,6 +25,9 @@ export async function generateMetadata({ params }: { params: Promise<{ slug: str
 
 export default async function PostPage({ params }: { params: Promise<{ slug: string }> }) {
   const slug = (await params).slug
+
+  if (slug === '__no-posts__') notFound()
+
   const post = await getPost(slug)
 
   if (!post) notFound()