Is it valid for Stack Overflow data to have an answer that does not have an owner?

For example if you hit the following API url http://api.stackoverflow.com/1.1/answers/18, it does not bring back an owner. If I look at the quarterly data dump, the OwnerUserId field is null.

However if I go to the website and hit the question page (http://stackoverflow.com/questions/17/binary-data-in-mysql) I see a user (PHPGuy) who answered the question, but does not have a hyperlink for that user.

Would this be a user who no longer is part of Stack Overflow or does it mean something else?