Contact look-up for messages without a transport_type raises an exception.

Currently any attempt to display a message without a `transport_type` fails because the `delivery_class` look-up relies on a `transport_type` being set and contact look raises an error if the `delivery_class` is `None`.

Outbound messages are expected to not have a `transport_type` set when:

* They have yet to be dispatched towards a transport by the routing dispatcher because they are still in flight.
* Will never reach a transport because they conversation that sent them is not hooked up to one.