The behavior of a string variable with a null_string value is defined by the /vo2 compiler option:

With /vo2+ NULL_STRING is translated to String.Empty Without /vo2 NULL_STRING is translated to NULL

Calling the :ToString() method on a variable that has a null_string value throws an exception if the code is compiled without the /vo2 option.

Source: https://www.xsharp.info/forum/private-product/1392-null-string-and-the-x-runtime#10210