Fixed #25688 -- Made admin.register() disallow an empty list of models.

    from django.contrib.admin.sites import site, AdminSite

    def _model_admin_wrapper(admin_class):

        if not models:

            raise ValueError('At least one model must be passed to register.')

        admin_site = kwargs.pop('site', site)

        if not isinstance(admin_site, AdminSite):

    def test_custom_site_not_an_admin_site(self):

        self.assertRaisesMessage(ValueError, 'site must subclass AdminSite',

            register(Person, site=Traveler), NameAdmin)

    def test_empty_models_list_registration_fails(self):

        with self.assertRaisesMessage(ValueError, 'At least one model must be passed to register.'):

            register()(NameAdmin)