exports: test if a join is already loaded

when a specific join has to be shared between combined filters/aggregators, we need to check if it has been already loaded.
we cannot load it into indicator because result is wrong

src/Bundle/ChillPersonBundle/Export/Export/CountHousehold.php

@@ -107,7 +107,6 @@ class CountHousehold implements ExportInterface, GroupedExportInterface
             ->join('acppart.person', 'person')
             ->join('person.householdParticipations', 'householdmember')
             ->join('householdmember.household', 'household')
-            ->join('household.compositions', 'composition')
         ;
 
         $qb->select('COUNT(DISTINCT householdmember.household) AS export_result');