Sending 0-length data must not result in a message

Passing 0-length data to enm:send() now just returns without sending an
empty message to the receiver(s).
Expand enm_eqc testing

Expand the enm_eqc test by introducing generators for protocol,
transport, mode, message content, and active setting for the receive
socket. This is an incremental step to expand the testing.  The
protocol generator still only uses pipeline so it serves primarily as
a placeholder.