Sitemap empty

Good morning.

I discovered this problem: I installed “Yoast SEO plugin”, I realized that by clicking on http://www.festeaziendaliaroma.com/sitemap_index.xml, a page is visibly empty.

Do you know tell me why?

I state that in Google web master these sitemaps are

/ attachment-sitemap.xml
/ category-sitemap.xml
/ post-sitemap.xml
/ sitemap_index.xml

with inside pages and indexed images.

thanks

http://ww.wp.xz.cn/extend/plugins/wordpress-seo/