Handle nil values appropriately in approximately-equal

[galdre]

It's nice to be able to assert empty? on a coll that could be nil. It also seems we should be able to assert nil? on nil. Seemed like a bug!

(deftest failing-test
  (is (=? empty? nil)))
  
(deftest still-failing-test
  (is (=? nil? nil)))

The issue is that (= nil (type nil)).