Return empty, not null, a11y node in Workspace

It's not valid to return a null accessbilitynode.

Test: local
Bug: 182529089

Change-Id: I94245fb33128c68e33ce388284f64e5ac3bb23dc

src/com/android/launcher3/Workspace.java

@@ -1455,7 +1455,7 @@ public class Workspace extends PagedView<WorkspacePageIndicator>
             // TAPL can work only if UIDevice is set up as setCompressedLayoutHeirarchy(false).
             // Hiding workspace from the tests when it's
             // IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS.
-            return null;
+            return AccessibilityNodeInfo.obtain();
         }
         return super.createAccessibilityNodeInfo();
     }