Why Django stops execution if reverse relation does not exist?

I have a common situation: a User model and an Employee model which is related to User by one-to-one relation on user_id. My user model is customized and one of customizations is the method "is_employee":

class User(AbstractUser):
    def is_employee(self):
        print(self.id)
        print(self.employee is not None)
        return self.employee is not None
    is_employee.boolean = True
    is_employee.short_description = _('Employee status')


class Employee(AddressMixin, models.Model):
    user        = OneToOneField(User, 
                                primary_key=True, 
                                related_name='employee',
                                on_delete=CASCADE)
    phone_no    = models.CharField(max_length=20, 
                                   blank=True, 
                                   default='')
    mobile_no   = models.CharField(max_length=20, 
                                   blank=True, 
                                   default='')

I use User.is_employee in the admin site on list_display. The problem is that the function User.is_employee only returns a result if there is an Employee for that user. If there is not then it stops executing where it first meets the call self.employee ant thereby returns None. And here is what I see on the admin: Employee status for the last user last user is not False but None

Sorry if the question was messy. I am quite new to Django and this is my first post on this site as well. Thank you in advance.