more robust macro implementation, test for edge case

fixes #265

Author: mpollmeier

gremlin-scala/src/test/scala/gremlin/scala/marshallable/MarshallableSpec.scala

@@ -351,6 +351,11 @@ class MarshallableSpec extends WordSpec with Matchers {
     results shouldBe Set(cc1, cc2)
   }
 
+  case class EmptyCC()
+  "handle case class without elements" in new Fixture {
+    val m = implicitly[Marshallable[EmptyCC]]
+  }
+
   trait Fixture {
     val graph = TinkerGraph.open.asScala
   }

macros/src/main/scala/gremlin/scala/Marshallable.scala

@@ -196,7 +196,10 @@ object Marshallable {
                 $label,
                 _root_.scala.collection.immutable.List(..$fromCCParams)
                   .flatten
-                  .filter(kv => _root_.scala.Option(kv._2).isDefined)
+                  .filter { kv =>
+                    kv.isInstanceOf[Product2[_, _]] &&
+                    kv.asInstanceOf[Product2[_, _]]._2 != null
+                  }
               )
             def toCC(element: _root_.gremlin.scala.Element): $tpe = $companion(..$toCCParams)
           }