In Excel, you can delete contents of a cell with data validation. From a user perspective, this is a combobox with all selected options, and a blank item. To set an empty value, the web application requires an actual blank item. If the validated cell is blank and there are no blank items in the validation list, SpreadsheetWEB wil automatically add the extra blank item and prevent this issue.
Although it is rare, combobox value can sometimes return an empty item after calculation. This is more likely to occur in dynamic (updated by a formula) lists. As a workaround, you can add an empty cell in the data validation range.