core: Allow the motor position to be null

Closes daiquiri-tomo#223 (closed)

I think it would be better to consider such case as an offline motor.

Because every attributes could be null, but at some point we have to distinguish what is valid and what it's not.

Discarding the device as offline or invalid would let us to handle it earlier and in a better and easier way.

Most of the changes are tomo related, so i am fine with it.