Materialized Paths Exclude Empty Objects and Arrays

The assertion in this program fails:

``` scala
import ahlers.michael.playful.json.JsValues._
import play.api.libs.json.Json._
import play.api.libs.json._

val expected =
  __ \ 'empty \ 'object -> obj() ::
    __ \ 'empty \ 'array -> arr() ::
    Nil

val actual = materialized(obj("empty" -> obj("object" -> obj(), "array" -> arr())))

assert(actual == expected)
```
