procedure VarClear(var Value: Variant)

 The Null value is a special variant value that indicates that a given variant does not have a value (or type). It is different from a nil value that is used with object instances, class types, method and routine references, and arrays.

var
   MyValue: Variant;
begin
   MyValue := 'Hello World';
   X := VarNull(MyValue);  // X is False
   VarClear(MyValue);
   X := VarNull(MyValue);  // X is True
end