Fix comparison with Version::NULL on RHS

This was accounted for in Version::NULL.<=>, but not in Version#<=>,
so these could only be compared if Version::NULL was the left hand
side.

The check had to go above the check that `other` is a version because
`Version::NULL`'s anonymous class does not inherit from `Version`.
(The type check feels like it's probably a smell, but out of scope).