Creates a property with a given name and sets its value to null.
This method is useful for those situations where you know at compile time that you will be adding a JSON null value. To use one of the Add( ) overloadings, you must set a variable to the Unknown value (?) first, then call Add( ) with the variable. Using the Unknown value (?) value as a constant in one of the Add( ) overloadings causes a compilation error, because the compiler is unable to resolve the overloading to use.
On successful execution, this method returns TRUE.
