Check null taskInfo for occlude-by-dream

Another instance of I479b05a5da896a166a375d30fdf050dc87448f3c
specific to the transition/remote animator for dream starting
over keyguard.

Change-Id: I731f5f10abfcc47e8a54bfa07445f25c2eb977b0
Test: atest android.platform.test.scenario.sysui.lockscreen
Fix: 283335509