handle None position sometimes returned by undulator revolvers.
It happens that due to a temporary communication error with undulator PLC, the position attribute quality factor remains INVALID for up to 1s. 1s is the refreshing period of the tango server low level acquisition thread that checks state and position of revolver axis. If the attribute position is read during that 1s period, None is returned as a position, what leads to typeError exception in bliss, while the next position readout would/could be ok.