Guard absent SplitScreen in PipTaskOrganizer

Current code assumes the presence of SplitScreen, so it crashes if
SplitScreen doesn't exist.

Bug: 207495281
Test: Transition in and out of PIP still works on phones.
Test: Transition from PIP to split screen on phones.
Change-Id: I6f2f05a1237c9e64f1df95b05347a1affa9b5a27

libs/WindowManager/Shell/src/com/android/wm/shell/pip/PipTaskOrganizer.java

@@ -1282,6 +1282,9 @@ public class PipTaskOrganizer implements ShellTaskOrganizer.TaskListener,
     }
 
     private boolean isPipTopLeft() {
+        if (!mSplitScreenOptional.isPresent()) {
+            return false;
+        }
         final Rect topLeft = new Rect();
         final Rect bottomRight = new Rect();
         mSplitScreenOptional.get().getStageBounds(topLeft, bottomRight);