lttng: Enable null-checking in test plugins

lttng: Enable null-checking in test plugins

For some reason, annotation-based null checks were disabled in the
tests plugins. To avoid NPEs during tests, might as well enable
and benefit from them.

Change-Id: Ib5b50a37e477aa8619f316cdbf0271ab33fce796
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/26072
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Hudson CI