Add null check for full-backup task

The backup-already-running code-path leaves the field null.

Bug: 112359186
Test: atest KeyValueBackupTaskTest
Test: 1. adb shell bmgr backupnow --all &>/dev/null&
      2. adb shell bmgr backupnow com.google.android.apps.backupapp
      3. Verify it finishes and no error

Change-Id: I6880c4e7752b943608d41c946e8d18d46cd9da38