compatibilityversion 8 or later, the
Vortex variable has a different meaning - it is a variable that is
permanently zero-valued (i.e. a constant of no values). It also loses
its special meaning as the default for the
<SQL NULL> option.
See here for details.
Caveat: Assignments to $null will cause a runtime error, and any usage of $null by <SQL> must be changed to the <SQL NULL> option, when compatibilityversion is 8 or later.