src.ctf.lttng-live: use optional pattern for `ctf_stream_class_id`

src.ctf.lttng-live: use optional pattern for `ctf_stream_class_id`

Instead of checking for an initial boundary value to signify if the
value is initialized use a optional like anonymous struct with a
`is_set` flag.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I3045ef884345bea58824fd2170d29e982d219b7a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6699
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>