Currently, when you group by a Custom Field in List view, if there are no results for a particular field option, the Custom Field option does not show.
This is not useful if you like to move things between custom fields by dragging tasks.
For example, if you have a List grouping by a Dropdown Custom Field for each day of the week, but you have no items with the option "Thursday", Thursday simply does not show on the List.
It would be useful to be able to toggle empty Custom Field groups on, so that Thursday could be shown.
You can do this with empty Statuses and dates, but not Custom Fields.
You can also show empty columns on Board view when grouping by a Custom Field, but not List view.