Set all attributes and unset attributes by using a value of 'null'.

def update_user_attributes(self, id, old_attrs, new_attrs):
        """
        Set all attributes and unset attributes by using a value of 'null'.
        """
        from alerta.utils.collections import merge
        merge(old_attrs, new_attrs)
        attrs = {k: v for k, v in old_attrs.items() if v is not None}
        update = {
            '$set': {'attributes': attrs}
        }
        response = self.get_db().users.update_one({'_id': {'$regex': '^' + id}}, update=update)
        return response.matched_count > 0