In the user facing filters for a blog posts block, limit the contributors shown as available option to select to only contributors that have an associated blog post.

Right now the contributor drop down shows all contributors whether they've written a blog or not. We use contributors for lots of things, not just blog authors. So in our case most contributors selected would show no results for blog posts. Simplifying that list to only the contributors that have written a blog would make it clearer for our users.