Check null taskInfo for occlude transitions

KeyguardService.wrap() is liberal with what it counts as an "app". The
first item in the apps list might not have a taskInfo if it's not a
Task, which causes a crash when unlocking the device for the first time.

Change-Id: I479b05a5da896a166a375d30fdf050dc87448f3c
Test: atest android.platform.test.scenario.sysui.lockscreen
Bug: 274954192