Skip foregound check if ActivityManagerService returns null list

In some unknown cases, ActivityManagerService can return a null list
of processes. This is already a corner case, and we do have other
checks so it should be safe to return `true` when this happens.

Fixes: 144821633

Test: BiometricPromptDemo
Change-Id: I07458f812be7f8e331e48cd5566568ed015596bd