If a device has been removed from one of the rings, it actually is set as None
within the ring. In that case the length of the devices is not True without
filtering the None devices. However, if the length matched the condition but
included a removed device the probetests would fail with a TypeError.
This fix could be done also in swift/common/ring/ring.py, but it seems it only
affects probetests right now, thus fixing it there and not changing the current
behavior.
Change-Id: I8ccf9b32a51957e040dd370bc9f711d4328d17b1