[spark-4691][shuffle]code optimization for judgement

In HashShuffleReader.scala and HashShuffleWriter.scala, no need to judge "dep.aggregator.isEmpty" again as this is judged by "dep.aggregator.isDefined"

In SortShuffleWriter.scala, "dep.aggregator.isEmpty"  is better than "!dep.aggregator.isDefined" ?


could change this guy to a require as well, and maybe change the message too