Update sitemap module sitemap.php

This fix is included to solve this bug: https://github.com/pyrocms/pyrocms/issues/2838
Now the function check if there are posts with status "Live", if the count is 0 then the loop doesn't print the blog link to the sitemap
Merge pull request #2872 from ChristianGiupponi/patch-2

Update sitemap module sitemap.php mkay?