Fix errors related with distributed.worker.memory.terminate

[pentschev]

None is not a valid value for distributed.worker.memory.terminate anymore, we should use False instead.

Errors such as the one below are raised otherwise:

2022-04-25 13:42:48,943 - tornado.application - ERROR - Exception in callback functools.partial(<bound method NannyMemoryManager.memory_monitor of <distributed.worker_memory.NannyMemoryManager object at 0x7fd3de281b50>>, <Nanny: tcp://127.0.0.1:39989, threads: 1>)
Traceback (most recent call last):
  File "/datasets/pentschev/miniconda3/envs/tmp/lib/python3.8/site-packages/tornado/ioloop.py", line 905, in _run
    return self.callback()
  File "/datasets/pentschev/miniconda3/envs/tmp/lib/python3.8/site-packages/distributed/worker_memory.py", line 326, in memory_monitor
    if memory / self.memory_limit > self.memory_terminate_fraction:
TypeError: '>' not supported between instances of 'float' and 'NoneType'

[quasiben]

@gpucibot merge

[codecov-commenter]

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[jakirkham]

rerun tests

[pentschev]

Thanks @quasiben and @jakirkham !