Add empty-mix test for clustering
Add empty-mix test for clustering

roselleebarle04/jubatus,roselleebarle04/jubatus,jubatus/jubatus,gintenlabo/jubatus,jubatus/jubatus,elkingtonmcb/jubatus,mathn/jubatus,Asuka52/jubatus,gintenlabo/jubatus,rimms/jubatus,rimms/jubatus,kmaehashi/jubatus,Asuka52/jubatus,mathn/jubatus,kmaehashi/jubatus,elkingtonmcb/jubatus,jubatus/jubatus