Initialize numNonEmptyBlocks in HighlyCompressedMapStatus.readExternal

In Scala 2.13, this causes several UT failures because `HighlyCompressedMapStatus.readExternal` doesn't initialize this field. The following two are the examples.

• org.apache.spark.rdd.RDDSuite

    Cause: java.lang.NoSuchFieldError: numNonEmptyBlocks
    at org.apache.spark.scheduler.HighlyCompressedMapStatus.<init>(MapStatus.scala:181)
    at org.apache.spark.scheduler.HighlyCompressedMapStatus$.apply(MapStatus.scala:280)
    at org.apache.spark.scheduler.MapStatus$.apply(MapStatus.scala:73)
    at org.apache.spark.shuffle.sort.UnsafeShuffleWriter.closeAndWriteOutput(UnsafeShuffleWriter.java:231)
    at org.apache.spark.shuffle.sort.UnsafeShuffleWriter.write(UnsafeShuffleWriter.java:179)
    at org.apache.spark.shuffle.ShuffleWriteProcessor.write(ShuffleWriteProcessor.scala:59)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:99)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:52)
    at org.apache.spark.scheduler.Task.run(Task.scala:127)
    at org.apache.spark.executor.Executor$TaskRunner.$anonfun$run$3(Executor.scala:464)
  

• org.apache.spark.scheduler.MapStatusSuite

  *** RUN ABORTED ***
    java.lang.NoSuchFieldError: numNonEmptyBlocks
    at org.apache.spark.scheduler.HighlyCompressedMapStatus.<init>(MapStatus.scala:181)
    at org.apache.spark.scheduler.HighlyCompressedMapStatus$.apply(MapStatus.scala:280)