CORE-3858 Fixes for DataMainPanel::GridDropTargetImpl.

Updated GridDropTargetImpl to validate the drop targets on dragEnter as well as dragMove, since drag events can go from dragEnter to dragDrop without going through dragMove in-between.
Added some null and empty list checks to GridDropTargetImpl and the DiskResourceServiceFacade::moveDiskResources call for safety.(cherry picked from commit f2f123a96d62114f99d4f6694cf5652767f7b2a1)