Fix empty relation when kill executors

[ulysses-you]

Why are the changes needed?

This pr fixes a corner case when repartition on a local relation. e.g.,

Repartition
      |
LocalRelation

it would throw exception since there is no an actually shuffle happen

java.util.NoSuchElementException: key not found: 3
	at scala.collection.MapLike.default(MapLike.scala:235)
	at scala.collection.MapLike.default$(MapLike.scala:234)
	at scala.collection.AbstractMap.default(Map.scala:63)
	at scala.collection.MapLike.apply(MapLike.scala:144)
	at scala.collection.MapLike.apply$(MapLike.scala:143)
	at scala.collection.AbstractMap.apply(Map.scala:63)
	at org.apache.spark.sql.FinalStageResourceManager.findExecutorToKill(FinalStageResourceManager.scala:122)
	at org.apache.spark.sql.FinalStageResourceManager.killExecutors(FinalStageResourceManager.scala:175)

How was this patch tested?

• Add some test cases that check the changes thoroughly including negative and positive cases if possible

• Add screenshots for manual tests if appropriate

• Run test locally before make a pull request

[ulysses-you]

thanks, merging to master