In the current code, when both partitions happen to have zero-length, the return mean will be NaN.
Consequently, the result of mean after reducing over all partitions will also be NaN,
which is not correct if there are partitions with non-zero length. This patch fixes this issue.
fix block manager UI display issue when enable spark.cleaner.ttl

Conflicts:

	core/src/main/scala/spark/storage/StorageUtils.scala