fix: do not generate rss if no valid posts

pages/blog/[...slug].js

@@ -32,8 +32,10 @@ export async function getStaticProps({ params }) {
   const authorDetails = await Promise.all(authorPromise)
 
   // rss
-  const rss = generateRss(allPosts)
-  fs.writeFileSync('./public/feed.xml', rss)
+  if (allPosts.length > 0) {
+    const rss = generateRss(allPosts)
+    fs.writeFileSync('./public/feed.xml', rss)
+  }
 
   return { props: { post, authorDetails, prev, next } }
 }