YARN-1128. FifoPolicy.computeShares throws NPE on empty list of Schedulables (Karthik Kambatla via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1525316 13f79535-47bb-0310-9956-ffa450edef68