My experience is that the AI Formula Bot is not overly helpful with dropdown conditions.
In a quick test, a Reference column pointing to a column with empty cells does not provide a blank option in the dropdown list. However, if you are seeing this behavior, if you review the help docs for Reference columns: https://support.getgrist.com/col-refs/#filtering-reference-choices-in-dropdown-lists you’ll find that you have to refer to the choice attribute for your reference column. It would look something like:
