`wp_dropdown_pages()` allows `option_none_value` to be passed. Add that arg to the `$defaults` for `wp_dropdown_users()` and `wp_dropdown_categories()` as well.

Props solarissmoke. 
Fixes #16625.


git-svn-id: https://develop.svn.wordpress.org/trunk@28564 602fd350-edb4-49c9-b593-d223f7449a82