User form cannot be submitted without setting an openerp user

openerp.module adds fields to the user profile form to allow to assign an openerp user to a Drupal user account. When submitting the from without an openerp user name, the form validation fails and the form can't be submitted. Attached patch makes it optional to enter something in the user name field, making the assignment of an openerp user to a Drupal user account optional.

Only two lines are changed, the patch looks bigger than it is because it includes some spacing changes (tabs replaced by spaces).