[2.2, 2.3] RegionService can raise unexpected exceptions for an empty connection list

The symptom can be seen as follows:

2017-07-18 18:11:46 maasserver.utils.views: [error] 500 Internal Server Error @ /MAAS/rpc/
Traceback (most recent call last):
  File "/usr/lib/python3.5/random.py", line 253, in choice
    i = self._randbelow(len(seq))
  File "/usr/lib/python3.5/random.py", line 230, in _randbelow
    r = getrandbits(k) # 0 <= r < 2**k
ValueError: number of bits must be greater than zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 108, in get_response
    response = middleware_method(request)
  File "/usr/lib/python3/dist-packages/maasserver/middleware.py", line 160, in process_request
    self._check_rack_controller_connectivity()
  File "/usr/lib/python3/dist-packages/maasserver/middleware.py", line 133, in _check_rack_controller_connectivity
    connected_ids = list(client.ident for client in getAllClients())
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 169, in wrapper
    return func_in_reactor(*args, **kwargs).wait(LONGTIME)
  File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
    result.raiseException()
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 368, in raiseException
    raise self.value.with_traceback(self.tb)
IndexError: Cannot choose from an empty sequence