See

https://eu.swi-prolog.org/pldoc/man?section=standardorder

for the "Standard Order of Terms".

Beware the types:

?- compare(D,null,"null").
D =  (>).

?- compare(D,"null",null).
D =  (<).

?- compare(D,null,null).
D =  (=).

?- compare(D,"null","null").
D =  (=).

You may want to use assertion/1 to check your code for surprises here.

This I consider crazy (sorting by the names of variables, because logically compare/3 shouldn't have any access to those variable names; variable names are a feature of the source code, not of the process active at runtime). What should be returned here? Equality of course. These are unbound variables!

?- compare(D,X,Y).
D =  (<).

?- compare(D,Y,D).
D =  (>).

?- compare(D,X,X).
D =  (=).