Fix: error when listing sessions with no session

Fix: error when listing sessions with no session

lttng_list_sessions() returns a "fatal error" code when
lttng_ctl_ask_sessiond() returns 0. This was interpreted as the
control socket being shutdown unexpectedly. However, it is
(more often) caused by no sessions being available. Given that, it
makes more sense to report that no sessions are available.

More clean-up/refactoring would be needed to report unexpected socket
shutdowns.

Fixes #1188

Signed-off-by: Geneviève Bastien <gbastien@versatic.net>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>