Pretty Printing of 'nil'

Fix for Exception thrown when trying to `pprint` a value containing `nil`:

```clojure
(fipp.edn/pprint nil)
; => IllegalArgumentException No implementation of method: :-pretty of 
;    protocol: #'fipp.edn/IPretty found for class: nil  clojure.core/
;    -cache-protocol-fn (core_deftype.clj:541)

(fipp.edn/pprint {:a nil})
; => IllegalArgumentException No implementation of method: :-pretty of 
;    protocol: #'fipp.edn/IPretty found for class: nil  clojure.core/
;    -cache-protocol-fn (core_deftype.clj:541)
```

(Broke some code for me after switching from `0.1.0-SNAPSHOT` to `0.3.0-SNAPSHOT`.)