下拉列表中的空白选项从自定义Sharepoint列表中消失

I have a custom list template that is deployed as a .wsp. The list has almost 30 columns, including several DropDownLists of column type "Choice". None of the columns are required and the customer wants a blank option as the default selection for each. Unfortunately once the list item has been saved, any columns that have had values selected can no longer be un-selected.I have a custom list template that is deployed