Add null check in BackupManagerService$PerformBackupTask.handleCancel()

handleCancel() can be called before we start backing up any particular
package, therefore we need to properly handle log messages in that case.

Bug: 36638539
Test: manual
Change-Id: Ic96e2fbad5c0a79640a3419d187e8b5e4d265de3
(cherry picked from commit 519a87db)