Bypass 0 byte allocation when no domains are enabled

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: Don't report an error when listing a session with no channels

The LTTng sessiond daemon reports an error when the list_channel
command is invoked on a session which only has a metadata channel.

The error lies in assuming that a session which has an enabled
domains also has (user-visible) channels.

The metadata channel should be listed in a future fix.

Fixes #855

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>