Merge r1555975 from trunk. YARN-1560. Fixed TestYarnClient#testAMMRTokens failure with null AMRM token. (Contributed by Ted Yu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1555977 13f79535-47bb-0310-9956-ffa450edef68