LU-5005 utils: handle ROOT path correctly

MDT replies path '\0' to the request of fid2path on ROOT,
client utils should handle it properly.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ic7064866b0e8ef33fc0a17e273a120d6d6dae4fe
Reviewed-on: http://review.whamcloud.com/10211
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>