Sitemaps: Ensure entry for ‘page’ post type sitemap in index.

If there are no pages and no static homepage, there will still be one sitemap including the homepage URL.

This change ensures that this sitemap is correctly listed in the sitemap index.

Props Chouby, pacifika, elrae.
Fixes #50571.
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git-svn-id: https://core.svn.wordpress.org/trunk@48245 1a063a9b-81f0-0310-95a4-ce76da25c4cd