gistfile1.scala

scala> import argonaut._, Argonaut._
import argonaut._
import Argonaut._

scala> Map( "foo" -> List("bar","baz") , "test" -> List() ) .jencode.spaces2
res7: String =
{
  "foo" : [
    "bar",
    "baz"
  ],
  "test" : [

  ]
}