Workflow Types; Items; Summary page showing "NaN" for HTML Widget parameters

The Summary Page below the Items Treeview on the Workflow Types form was showing "NaN" as the value for HTML Widget Parameters if the "Customise HTML Widget's Parameters" checkbox was checked.

This was due to the business layer ISKeyValueListItem.HasValue() method incorrectly returning True for a "Double" type item that, instead of having a value of zero, had a value of Double.NaN (Not a Number).