[SPARK-13441][YARN] Fix NPE in yarn Client.createConfArchive method

## What changes were proposed in this pull request?

Instead of using result of File.listFiles() directly, which may throw NPE, check for null first. If it is null, log a warning instead

## How was the this patch tested?

Ran the ./dev/run-tests locally
Tested manually on a cluster

Author: Terence Yim <terence@cask.co>

Closes #11337 from chtyim/fixes/SPARK-13441-null-check.