[{"key":1,"localTimestamp":undefined,"exampleArray":[],"exampleField":false,"status":"active"},{"key":2,"localTimestamp":undefined,"exampleArray":["bla"],"exampleField":false,"status":"pending"},{"key":3,"localTimestamp":undefined,"exampleArray":[""],"exampleField":false,"status":"pending"}]

db.collection.update({status:"active"},[{$set:{exampleField:"works",localTimestamp:{$cond:{if:{$eq:[{$size:"$exampleArray"},0]},then:new Date(),else:"$localTimestamp"}}}}])