Fix NPE occured when null ClipData is passed to startDrag.

Previously we have NPE for the following cases:

 * when specifying both DRAG_FLAG_GLOBAL and DRAG_FLAGS_URI_ACCESS, OR
 * when the userIds of source and target apps are different

Bug: 70477060
Test: com.android.server.wm.DragDropControllerTests,
      android.server.wm.CrossAppDragAndDropTests,
      manually check the drag and drop behavior on test app.
Change-Id: I454b434e4466cb159ce23e0982012cdb21c9d002