If a post has no owner for whatever reason, we don't get the owner object in th response.

Thus, it is impossible to display the username of the missing owner. The site is able to do this, so we should be given the name of the user as well.

See meta question #33809 for a view of all of these scenarios.

Here is an example request returning now owner at all:

http://api.meta.stackoverflow.com/1.0/answers/33812