If a post has no owner for whatever reason, we don't get the owner object in the 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:
I feel that this is very important, othwerwise we are unable to attribute a post to an author.
user_idis spec'd as a required value on the returned "user object" and the root problem here is that the user was deleted and thus has no id.