#137 Lightweight dummy port

[Tom Smith]

In cases where multiple machine constructors exist, different constructors may initialize different ports. If a port is reference is null, and the machine enters a state where a transition is defined on that port, then the ECharts runtime raises a MachineException: "Null port specified for message transition."

It is a waste of resources to create, for example, s SipPort that won't be used in a certain scenario (this class contains a bunch of fields, including a SipPortFSM). Consider adding a class to support a "dummy port." It could even have the SipPort API (if SipPort is made into an interface) but a do-nothing implementation,

[Gregory W Bond]

perhaps should be a Null SIPPort - SipPort constructor arg to indiciate that it's a null port - what are consequences for ECharts when a null SIP port is in an active state?