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.
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