HADOOP-18402. S3A committer NPE in spark job abort (#4735)

JobID.toString() and TaskID.toString() to only be called
when the IDs are not null.

This doesn't surface in MapReduce, but Spark SQL can trigger
in job abort, where it may invok abortJob() with an
incomplete TaskContext.

This patch MUST be applied to branches containing
HADOOP-17833. "Improve Magic Committer Performance."

Contributed by Steve Loughran.
2 files changed