SortableList allows dragging of EmptyItem

[mhspelt]

I have two SortableLists in the same group, with the idea of dragging items between them.

There is an EmptyDataTemplate that shows when one of the lists is empty.

I can take the empty data block and drag it to the other list, and this will fire OnRemove on the empty list.

Of course, this is easy to catch, but in my opinion I shouldn't be able to remove "no item" from an empty list at all.