Featured Content: Correct check for whether or not the front page is the blog page.

Checking if `page_on_front` is empty can cause problems, since it's no longer empty as soon as a page is set once - even when the user changes back to using the blog as the front page, `page_on_front` retains the page ID. `show_on_front` is a more accurate way of determining if the blog is the front page or not.
Merge pull request #1098 from Automattic/fc-pre-get-posts

Featured Content: Correct check for whether or not the front page is the...