incorrect warning about missing afferent projections

[jvesely]

The warning:

$HOME/PsyNeuLink/psyneulink/core/compositions/composition.py:6572: UserWarning: InputPort ('InputPort-0') of 'ProcessingMechanism-0' doesn't have any afferent Projections.

appears even for nodes/mechanisms that don't have any inputs, (e.g. use PRNG functions). for example in test_modulation_of_random_state_direct:

        mech = pnl.ProcessingMechanism(function=pnl.UniformDist(seed=0))
        ctl_mech = pnl.ControlMechanism(control_signals=pnl.ControlSignal(modulates=('seed', mech),
                                                                          modulation=pnl.OVERRIDE))
        comp = pnl.Composition()
        comp.add_node(mech)
        comp.add_node(ctl_mech)