[{"mail":"sample@sample.com","name":"sample","businessesAllowed":[{"_id":"ENE","contractsAllowed":[{"_id":"TRA"}]}]}]

db.collection.aggregate([{$set:{clientsAllowed:{$cond:{if:{$and:[{"$eq":[{$type:"$businessesAllowed"},"array"]},{$ne:["$businessesAllowed",[]]}]},then:"SampleClientsAllowed",else:false}}}}])