Added a null value on None

public/biopet-framework/src/main/scala/nl/lumc/sasc/biopet/utils/ConfigUtils.scala

@@ -166,6 +166,7 @@ object ConfigUtils extends Logging {
   def anyToJson(any: Any): Json = {
     any match {
       case j: Json      => j
+      case None         => Json.jNull
       case m: Map[_, _] => mapToJson(m.map(m => m._1.toString -> anyToJson(m._2)))
       case l: List[_]   => Json.array(l.map(anyToJson(_)): _*)
       case n: Int       => Json.jNumberOrString(n)