[Dovecot] default_pass_scheme = PLAIN and blank password

I'm using Dovecot 1.2.15 on Ubuntu, but I think this applies to other versions as well. I store user info and passwords in MySQL. It appears 'default_pass_scheme = PLAIN' allows blank passwords. My question is: Is this a bug or is this by design? If it is by design, is there a setting that I'm not aware of that changes this behavior? Of course let me know if you need additional info on the setup I'm using.

Thank you, Gary V