on-event evaluation: introduce on-event evaluation specific status code

Before this change, `lttng_evaluation_on_event_get_captured_values`
returns `LTTNG_EVALUATION_STATUS_INVALID` when an evaluation's
condition has no capture descriptors. That status code is also used to
indicate a handful of "invalid parameter" conditions.

Not having captured values in an evaluation should not be considered an
error. Hence, a status enum that is specific to on-event evaluations is
added to introduce a "NONE" status that is not an error (positive value)
and wouldn't make sense for all evaluations (it could mean an error in
other circumstances).

`LTTNG_EVALUATION_ON_EVENT_STATUS_NONE` is returned when the condition
of an on-event evaluation has no capture descriptors.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ic2e3e90317a44c7bb3e95630956565690bffd760