Fixed #21517 -- Added unit test for non-autoincrement primary key with value 0.

            with transaction.atomic():

                Employee.objects.create(employee_code=123, first_name="Fred", last_name="Jones")

    def test_zero_non_autoincrement_pk(self):

        Employee.objects.create(

            employee_code=0, first_name="Frank", last_name="Jones"

        )

        employee = Employee.objects.get(pk=0)

        self.assertEqual(employee.employee_code, 0)

    def test_custom_field_pk(self):

        # Regression for #10785 -- Custom fields can be used for primary keys.

        new_bar = Bar.objects.create()