If all posts were skipped, remove the sitemap post.

[adamsilverstein]

Correct an issue where, when using the msm_sitemap_skip_post filter to exclude posts, all posts for a particular date happen to be excluded. In this case the plugin would generate an invalid/empty xml entry for the date. This fix instead skips the date/removes the sitemap cpt entry.

<urlset @xmlns:xsixmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:n="http://www.google.com/schemas/sitemap-news/0.9" xmlns:image="http://www.google.com/schemas/sitemap-image/1.1" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" />

The invalid sitemap then resulted in a google parsing error:

[adamsilverstein]

ps. you can reproduce the invalid xml issue by adding this code to a theme

add_filter( 'msm_sitemap_skip_post', '__return_true' );

...then regenerating the sitemaps. you will find empty sitemaps with invalid xml for dates where any post was published.