Business Layer; ISKeyValueList object; ToSimpleJsonString method; Now writes Number and Date Ranges as null

When calling the ToSimpleJsonString() method on the ISKeyValueList object, any Date Range or Number Ranges without values will be written as null.

Previously, these were written as their string serialisation which would typically result in a single pipe character as the value.

The HTML Widget widget.UI.ParametersHost control now also ensures that for Date Range and Number Range parameters, these produce a JavaScript object contain null values rather than a pipe character.