[Scheme] Use _type_predicate() for _op_null_p(). 
[Scheme]
Use _type_predicate() for _op_null_p(). 


git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@22694 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot