Return non-null empty list if the controller is already invalidated

- In the off chance that the controller is already invalidated (due to
  service reconnection to launcher for example), then any calls through
  the previous binder should just return an empty task list.

Bug: 206648922
Test: Manual (haven't been able to repro)
Change-Id: Iff6193fa89c2baecc527e589f17ae41b82575a48