I'm trying to add date of birth field in res.partner  and the date must be in the past

this is the code:

from odoo import models , fields,api,_
from odoo.exceptions import ValidationError
from odoo.exceptions import UserError

class ResPartner(models.Model):
    _inherit='res.partner'
    is_student=fields.Boolean(string="Is Student?")
    birth_date=fields.Date(string="Birth Date",rquired=True)


    @api.constrains('birth_date')
    def validation_constraints(self):
        today=fields.Date.today()
        for rec in self:

            if rec.birth_date>=today:
                raise ValidationError(_('Invalid date of birth ..please enter correct date'))

but when i create the user without insert birth date the error occurs:
'>=' not supported between instances of 'bool' and 'datetime.date'

How can i solve this problem when the user doesn't insert the date?