libbinder: clear RpcTransportCtx for new sessions in RpcServer

RpcServer previously created new RpcSession instances
using the default transport. There is currently no code
in RpcSession that uses that transport for server sessions,
so this patch initializes the transport to nullptr instead.

Bug: 224644083
Test: atest binderRpcTest
Change-Id: Ie9fa8c9c758821614fb161b360a9935955e7e62b