Fix birthdate for placeholder attendees
Fix birthdate for placeholder attendees

Forces blank birthdates to None value, which the database will accept,
in order to prevent 500 errors when saving placeholder attendees.

magfest/ubersystem,RAMSProject/rams,migetman9/ubersystem,md1024/rams,md1024/rams,magfest/ubersystem,magfest/ubersystem,migetman9/ubersystem,RAMSProject/rams,migetman9/ubersystem,magfest/ubersystem,RAMSProject/rams,md1024/rams