Current implementation moves to next agend group if current agenda group is empty. This is unexpected, and in part detrimental in the impl of units: it is not possible to suspend execution of rules by switching to an empty group, because the engine will move to the next group anyway:

return nextRule != null && (!ruleAgendaItem.getAgendaGroup().equals( nextRule.getAgendaGroup() ) || !isHigherSalience(nextRule));

because the change in behavior is potentially backwards incompatible we add a flag keepWhenEmpty that must be set to true when we want to enable this new behavior