Protocol extended to nil + satisfies? doesn't appear to work

``` clojure
user=> (defprotocol Foo)
Foo
user=> (extend-protocol Foo nil)
{:ns #object[sci.lang.Namespace 0x2d6c1717 "user"], :name user/Foo, :methods #{}, :var #'user/Foo, :sigs {}, :satisfies #{""}}
user=> (satisfies? Foo nil)
```