Fix: Handle undefined content in blog post rendering

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/[locale]/blog/[slug]/page.tsx

@@ -1382,7 +1382,7 @@ export default async function BlogPostPage({ params }: Props) {
         </div>
 
         {/* Content */}
-        {renderMarkdown(post.content)}
+        {renderMarkdown(post.content || '')}
       </article>
     </main>
   );