Qpid Cpp Messaging .NET Binding does not properly handle Qpid type VAR_VOID

Sending nullptr or receiving Qpid type VAR_VOID causes the binding to fail.

Sending nullptr should send a VAR_VOID value to Messaging.
Receiving a VAR_VOID should send a nullptr in place of an actual object to .NET binding clients.
The Message decoder will display a zero-length string when ToString() processes a nullptr.