Misbehavior of trees with empty nodes

[lukstafi]

While working on issue #26 I encountered the following bug / arguably undesirable behavior (to the left of subchild 4).
I would prefer that empty nodes are "transparent" to connectors: no up-tick toward an unrelated box.

let b =
  let open PrintBox in
  tree (frame @@ text "root") [
    frame @@ text "child 1";
    text "child 2";
    (* Note that an empty tree node causes a potentially-undesired but expected disconnection. *)
    frame @@ tree empty [
      tree (frame @@ text "header 3") [frame @@ text "subchild 3"]
    ];
    (* FIXME: empty tree nodes cause weird-looking connectors. *)
    tree empty [
      tree empty [frame @@ text "subchild 4"]
    ];
    frame @@ text "child 5"
  ]

let () = print_endline @@ PrintBox_text.to_string b

produces at head:

┌────┐
│root│
└────┘
├─┌───────┐
│ │child 1│
│ └───────┘
├─child 2
├─┌──────────────────┐
│ │└─┌────────┐      │
│ │  │header 3│      │
│ │  └────────┘      │
│ │  └─┌──────────┐  │
│ │    │subchild 3│  │
│ │    └──────────┘  │
│ └──────────────────┘
├─└─└─┌──────────┐
│     │subchild 4│
│     └──────────┘
└─┌───────┐
  │child 5│
  └───────┘