Incorrect behaviour when creating a PointNeuron with no synaptic mechanisms, with neuron backend

[apdavison]

When running a simulation with a composed neuron model with only a neuron component, no synapse components, the neuron backend produces incorrect behaviour.

Example of cell type:

cell = sim.PointNeuron(
    sim.AdExp(
        cm=123.5 * 1e-3,
        tau_refrac=2.47,
        v_spike=10,
        v_reset=-51,
        v_rest=-96.2,
        tau_m=3.5283698074395744, # 123.5 / (8.6 * 4.07)
        i_offset=0.0,
        a=-14.5,
        b=500.0 * 1e-3,
        delta_T=16,
        tau_w=15,
        v_thresh=-51,
    )
)

Expected behaviour (here using nest backend):

Actual behaviour:

Thanks to Ilaria Carannante for the bug report.