tmf.xml: Fix validate event in TmfXmlBasicTransition

tmf.xml: Fix validate event in TmfXmlBasicTransition

Previously, if the user do not specify an event in a transition, the
validation always yields false, which is not true. This patch returns
true instead.

Change-Id: I2ec0afb344c3e4fb51366168cca8ebac6c7cb30f
Signed-off-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/77272
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI