layouts: Allow blog posts without an author

This is needed by the blog post I'm adding in the next commit; it's
written by me it's really speaking for the company. Also all the
features it talks about are Will's ideas & Will's implementation so it's
a bit confusing to put my name to it.

Note that blog/atom.xml doesn't need this change because it doesn't tag
each entry with an author, it just has a global author for the whole
feed: "The stb-tester project".
blog post: Upcoming stb-tester features