Filter out empty register in NodeToJson converter

[TanUkkii007]

Overriding causally dependent primitive value with empty list or map creates an empty register in document state.

val p0 = Replica
    .empty("p")
    .applyCmd(doc.downField("key") := "A")
    .applyCmd(doc.downField("key") := `[]`)
    .applyCmd(doc.downField("key").iter.insert("A"))

MapNode(
  Map(
    MapT(DocK) -> MapNode(
      Map(
        RegT(StrK(key)) -> RegNode(Map()),
        ListT(StrK(key)) -> ListNode(
          Map(
            RegT(IdK(Id(3,p))) -> RegNode(Map(Id(3,p) -> Str(A)))
          ),
          Map(IdK(Id(3,p)) -> Set(Id(3,p))),
          Map(HeadR -> IdR(Id(3,p)), IdR(Id(3,p)) -> TailR)
        )
      ),
      Map(StrK(key) -> Set(Id(2,p), Id(3,p)))
    )
  ),
  Map(DocK -> Set(Id(1,p), Id(2,p), Id(3,p)))
)

Converting documents that contain empty resister with last-writer-wins conflict resolver causes Exception: java.lang.UnsupportedOperationException: empty.max

This is because the empty register is viewed present as presSets has key "key" so JSON converter tries to convert the empty register to JSON value, which is impossible.

By the way Node#values query works correctly against empty register because it returns register's values that is empty. An implementation depends on Node states and does not use its query interface may hit this problem.