I'm guessing that WordPress has user_activation_key set to '' (empty string) when the user hasn't requested a password reset or similar email-confirmed operatio

I'm guessing that WordPress has user_activation_key set to '' (empty string) when the user hasn't requested a password reset or similar email-confirmed operation. So, it checks to see if it is empty, it isn't and so it checks to find the user(s) who have empty user_activation_key values. It's using a get_row() method which indicates that it will just return one and not an array which would have caused a type error. UPDATE: yes, user_activation_key is an empty string when WordPress hasn't received a request from a user to do something that would require it to be set.