Just return an empty list if the system returns null

Bug: 206648922
Test: Presubmit (unable to reproduce this)
Change-Id: I04a4759a1c237513ab314ece4acd542e1407edb1
diff --git a/quickstep/src/com/android/quickstep/SystemUiProxy.java b/quickstep/src/com/android/quickstep/SystemUiProxy.java
index d28796c..8b00f45 100644
--- a/quickstep/src/com/android/quickstep/SystemUiProxy.java
+++ b/quickstep/src/com/android/quickstep/SystemUiProxy.java
@@ -816,8 +816,12 @@
     public ArrayList<GroupedRecentTaskInfo> getRecentTasks(int numTasks, int userId) {
         if (mRecentTasks != null) {
             try {
-                return new ArrayList<>(Arrays.asList(mRecentTasks.getRecentTasks(numTasks,
-                        RECENT_IGNORE_UNAVAILABLE, userId)));
+                final GroupedRecentTaskInfo[] rawTasks = mRecentTasks.getRecentTasks(numTasks,
+                        RECENT_IGNORE_UNAVAILABLE, userId);
+                if (rawTasks == null) {
+                    return new ArrayList<>();
+                }
+                return new ArrayList<>(Arrays.asList(rawTasks));
             } catch (RemoteException e) {
                 Log.w(TAG, "Failed call getRecentTasks", e);
             }