[SPARK-4691][shuffle] Restructure a few lines in shuffle code

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" ?

Author: maji2014 <maji3@asiainfo.com>

Closes #3553 from maji2014/spark-4691 and squashes the following commits:

bf7b14d [maji2014] change a elegant way for SortShuffleWriter.scala
10d0cf0 [maji2014] change a elegant way
d8f52dc [maji2014] code optimization for judgement

(cherry picked from commit b31074466a83d3d1387fc1e4337dfab9e164fc04)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>