Add null check for invalid previously appeared target

* In ag/23680185, a null check was removed that was
checking if individual mLastAppearedTaskTargets were null;
we check if the array overall is null, but individual
elements can also be null, ex 3P launcher

Bug: 289609734
Test: Repro steps from b/289609734 don't cause crash
Flag: none
Change-Id: Iddfde6d9ac2b708380b70b5fb6301b629506619c