Как вернуть пустое значение?

def get_profile_cover_picture(self):
  default_cover = None
    if self.cover:
      return self.cover.url
    else:
      return default_cover

def get_profile_cover_picture(self, obj):
	request = self.context.get('request')
	profile_cover_picture_url = obj.get_profile_cover_picture()
	return request.build_absolute_uri(profile_cover_picture_url)