Skip empty slice permissions files during backup

Test: adb shell bmgr backupnow --monitor-verbose android
  (make sure this finishes quickly & does not get stuck
  in an infinite loop)
Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/slice
Change-Id: Ieb701c7ed764c005c8a8517a9411edd8c36023f0
Fixes: 120904478