The incoming building was initialized like this:

Building building = null;
FactHandle buildingFact = businessRuleSession.insert( building );
businessRuleSession.setFocus("validate delete or get building");
businessRuleSession.fireAllRules();

Can drools test for this null building?
What should the rule look like?

rule "building is null"
    agenda-group "validate delete or get building"
    salience 10
    when
        boException : BOException()
        Building() == null /* this is the problem here -- not sure what this should look like */
    then
        boException.setCode("BO-0008");
end