Protect against weird null case

No time to figure out what's going on, but this null check helps

Test: Dock app, make fullscreen, hammer home button
Change-Id: I3eb94a8138bc1483b1711556696bcbbaee6084e7
Bug: 36339388
diff --git a/services/core/java/com/android/server/am/ActivityMetricsLogger.java b/services/core/java/com/android/server/am/ActivityMetricsLogger.java
index ebbce02..918747b 100644
--- a/services/core/java/com/android/server/am/ActivityMetricsLogger.java
+++ b/services/core/java/com/android/server/am/ActivityMetricsLogger.java
@@ -251,7 +251,8 @@
      *                       ActivityManagerInternal.APP_TRANSITION_* reasons.
      */
     void notifyTransitionStarting(SparseIntArray stackIdReasons) {
-        if (!isAnyTransitionActive() || mLoggedTransitionStarting) {
+        // TODO (b/36339388): Figure out why stackIdReasons can be null
+        if (stackIdReasons == null || !isAnyTransitionActive() || mLoggedTransitionStarting) {
             return;
         }
         mCurrentTransitionDelayMs = calculateCurrentDelay();