YARN-786: Addendum so that RMAppAttemptImpl#getApplicationResourceUsageReport won't return null

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1543600 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-HADOOP-9166-branch-2-gd