Make 'Single directory suggestion' widget deal properly with directory entries without label

When the Single directory suggestion widget lists in its dropdown vocabulary entries that don't have label, it renders:
1. a line with label undefined if vocabulary entry was added with the widget option Add new entry

2. or improperly (half height line) an empty line (id vocabulary entry defined in Studio)

If there is no label, the widget should at least display the entry's ID so that the user can differentiate multiple entries withno label.