def empty(self):
        return None
L{Factory.doStop} logs that it is stopping a factory, followed by
dataset/ETHPy150Open codeinthehole/django-cacheback/cacheback/base.py/Job.empty--crossover with--dataset/ETHPy150Open twisted/twisted/twisted/internet/test/test_protocol.py/FactoryTests.test_doStopLoggingStatement