Fix RSS feed to handle nullable descriptions

The description field in blog/project schemas can be null,
causing RSS generation to fail. Added fallback to empty string.

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

src/pages/rss.xml.js

@@ -19,7 +19,7 @@ export async function GET(context) {
     site: context.site,
     items: items.map((item) => ({
       title: item.data.title,
-      description: item.data.description,
+      description: item.data.description ?? "",
       pubDate: item.data.date,
       link: `/${item.collection}/${item.id}/`,
     })),