Fix support for promise test less and anomaly less

• grid.promise: do no write execution timestamp if running testless or anomalyless promise

• grid.promise: send EmptyResult if promise is test less or anomaly less

There is now EmptyResult object which is an empty promise result. This result is send if the promise test or anomaly was skiped (if disabled). To prevent raise because EmptyResult is unknonwn on old slapos-node with slapos.core < 1.4.17 we will send an empty TestResult instead.