Add an empty element in *_getResourceItemList script, otherwise if the value is not set on the document, the listfield will display the first item, even though it's not really set on the document. This is confusing for users.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@19400 20353a03-c40f-0410-a6d1-a30d3c3de9de