fix: return empty object instead of null when no email

src/index.js

@@ -61,14 +61,14 @@ export default {
        LIMIT 1`
     ).bind(to).first();
 
-    if (!row) return json(null);
+    if (!row) return json({});
 
     if (Date.now() - row.received_at > ONE_DAY_MS) {
       // 读取时发现过期，立即删除，避免库里长期残留
       await env.DB.prepare(
         `DELETE FROM latest_emails WHERE recipient = ? AND received_at = ?`
       ).bind(row.recipient, row.received_at).run();
-      return json(null);
+      return json({});
     }
 
     return json({