'The last choice is for the None value.'

def test_choicesfieldlistfilter_has_none_choice(self):

    class BookmarkChoicesAdmin(ModelAdmin, ):
        list_display = ['none_or_null']
        list_filter = ['none_or_null']
    modeladmin = BookmarkChoicesAdmin(Bookmark, site)
    request = self.request_factory.get('/', {})
    changelist = self.get_changelist(request, Bookmark, modeladmin)
    filterspec = changelist.get_filters(request)[0][0]
    choices = list(filterspec.choices(changelist))
    self.assertEqual(choices[(-1)]['display'], 'None')
    self.assertEqual(choices[(-1)]['query_string'], '?none_or_null__isnull=True')
