Add a null check when getting TransportConnection in PFTBT

We currently call TransportManager#getCurrentTransportClient (marked as
@Nullable) in PerformFullTransportBackupTask without checking the result
for null. This causes occasional crashes (see attached bug).

Handle the case when TransportManager returns null.

Bug: 224056224
Test: 1. Manual: hardcode null return value in
         TransportManager#getCurrentTransport client, initiate a full backup,
         verify system_server does not crash)
      2. atest PerformFullTransportBackupTaskTest
Change-Id: Ic9fff3944cda712db4faa2f746449240558fd4d7