[BUG] Inconsistent error messages for empty stimulus

[mbeyeler]

Passing an empty stimulus to an implant & predicting a percept leads to inconsistent results, depending on how the stimulus is passed.

If called like this:

model = p2p.models.ScoreboardModel().build()
model.predict_percept(p2p.implants.ArgusII(stim=None))

the model will realize that there is no stimulus to be applied, so the output is None (no error thrown).

However, sometimes the stimulus is empty by accident, for example if dynamically built as a list or dictionary. Currently, passing stim=[] gives a "Number of electrodes provided (60) does not match the number of electrodes in the data (0)" error (which is fair, but inconsistent with the above) and passing stim={} gives a "Buffer has wrong number of dimensions (expected 2, got 1)" error (again, inconsistent).

All of these should lead to the same outcome, which is no error and None output.