It was the correct way (the transition Non was followed only once in fact). I've made a coding error later that made my trace be triggered twice.
So the correct way of doing a Decision in Java code is to code directly in the Source tab of your processdefinition.xml the following code :
<decision name="Client actif ?"> <handler class="fr.jl2tho.tutorialjbpm.action.IsCustomer" /> <transition name="Oui" to="Enregistrer la Question"></transition> <transition name="Non" to="Prevenir le commercial"></transition> </decision>