autonomous vehicles issued ChargingBaseInstruction when no stalls available

[robfitzgerald]

this is pretty benign as the effect is just that vehicles stay in a ReserveBase state. that said, it pukes up a lot of warnings to the user:

this happens because we are not checking charger availability in driver_instruction_ops.py::av_charge_base_instruction():

    chargers = tuple(filter(
        lambda c: my_mechatronics.valid_charger(c),
        [env.chargers[cid] for cid in my_station.total_chargers.keys()]
    ))