As I discovered in comment #58 in #2005970: In renderable arrays, #type should provide a #theme suggestion, similar to how drupal_prepare_form() works, drupal_get_css() will try to process an empty $css array when it could just return an empty string straight after the alter that allows modules to modify the array if it is still empty.
$css is generally only empty during ajax requests it seems.
Not returning early means it would be impossible for somebody to implement #theme 'styles' without causing exceptions on ajax requests, as the drupal_render() in drupal_get_css() would cause the theme system to initialise inappropriately.
Doing a quick empty() check would also be a slight performance boost for ajax pages.
This is probably minor in itself, but it's kind of blocking #2005970: In renderable arrays, #type should provide a #theme suggestion, similar to how drupal_prepare_form() works.