Fix issue #1068 - error in sitemap_index.xml if post type does not contain any posts

Also fixes a typo in the same line - this may well be the cause of the incorrect sitemap last modified dates as reported in #930, #883, #1066, #525, #873
Merge pull request #1093 from jrfnl/Issue-1068-and-possibly-more

Fix issue #1068 - error in sitemap_index.xml if post type does not ...