Can not move DraggableObject into SortableObjects if SortableObjects is empty

[mjzefeldt]

Not sure if this is intended aspect to feature or if there is a config that I am missing or if this is a bug.

• When I setup multiple SortableObject containers and the starting state is for a container to be empty (no DraggableObjects moved into it yet), I cannot move a DraggableObject into that SortableObject container.
• Similarly, if I move all of the DraggableObjects out of a SortableObject container, once the container is empty I can no longer move DraggableObjects back into the SortableObject container.
• You can see this behavior in the demos, for example: http://mharris717.github.io/ember-drag-drop/#/sort
  Move all of the Number Objects from one list to the other and you will not be able to move back into list once it is empty.