Fix NPE in BackupManagerService

If none of the requested packages are valid packages,
we'd run into this issue where PerformBackupTask#mQueue
is null but we try to iterate over it in finalizeBackup()
Fix is to use an empty queue as the default value instead
of null.

Bug: 35380067
Test: Manually tested by trying to backup a non-existent package.
Change-Id: Ibb2d03d5f8270bcb448ee083ee02964236aca92b