I am not skilled with freepascal or delphi, but I was playing around with this and tried to fix it or find the problem.
So I changed it:
var
s: specialize TNullable<string[8]>;
And got this error message:
project1.lpr(12,36) Error: Type parameters of specializations of generics cannot reference the currently specialized type
Then I looked it up in Google.
If it is not the same, forget it I do not pretend to understand it fully.