HDFS-4362. GetDelegationTokenResponseProto does not handle null token. Contributed by Suresh Srinivas.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1430137 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' of github.com:griddynamics/yhadoop-common into trunk-gd