See the linked Scene Builder issue : in case of empty list, a user may think "Empty" is an item that empty the ComboBox value, when it has no effect.
This is confusing. To avoid this confusion, the "Empty" label should be dimmed.

Proposed solution: add a new emptyText property. This is analogous with promptText, with the exception that emptyText is displayed as a grayed-out (and non-selectable) row in the ComboBox dropdown list when there are no other items in the items list.