Do not set outActivity if value is null.

Recent changes passed down the nullable array to a method where it
was not being properly checked.

Change-Id: Ibf940f9225e4c9901869188aa55a04d0908015c3
Fixes: 37197666
Test: manual via bug repro steps.

services/core/java/com/android/server/am/ActivityStarter.java

@@ -1038,7 +1038,7 @@ class ActivityStarter {
             sendPowerHintForLaunchStartIfNeeded(false /* forceSend */);
 
             reusedActivity = setTargetStackAndMoveToFrontIfNeeded(reusedActivity);
-            if (outActivity.length > 0) {
+            if (outActivity != null && outActivity.length > 0) {
                 outActivity[0] = reusedActivity;
             }