Easiest solution, provide a value of ' ' for the username field (which you can still call 'username').
If the value can be populated by user-inputted values, as is usually the case for a form that you are validating, provide a value of ' ' when it is not already set. In PHP,
if(trim($username) == ''){$username = ' ';}
I actually think autocompletion for username and password effectively gifts access to all your accounts to anyone who accesses your computer, regardless of how obscure your passwords are...
