Fix #256: loop for var nil works

(loop for var nil ...) should work the same as (loop for var of-type nil ...).

In loop-optional-type, there's a test for z. This is where the old-style type-specifier goes. We were failing here because when we try to find the type-spec via lookup in the loop universe, we fail. Instead, explicitly check for z being NIL. This causes us to pop the loop source to skip over nil. This is what happens if we had used fixnum instead of nil.