[KV] Only discard state if non-null

To avoid NPEs. Added tests. This design is not optimal but left any
re-design to be done when we move applyStateTransition() to more upper
levels, which is going to happen when we extract package-backup.

Bug: 117269444
Test: atest FrameworksServicesRoboTests
Test: 1. Set BackupApp to time-out on agent onCreate()
      2. adb shell bmgr backupnow --non-incremental
         com.google.android.apps.backupapp
      3. Verify it doesn't crash
Change-Id: I7a3fd3d3d5a4b5931206564c197edd86b6321933