[elk] hidden arrowheads

[bo-ku-ra]

https://play.d2lang.com/?script=vI4xysJAEIX7PcUjfYafFD8yha2FoBewmd1NQjDsQNSAyN5dduMcwMavmzcfj7fKcmM0scP6R_-0a_ByQOzaoGmYRq4nMMtTH_e2T-OUekY_Xx2QXXbufNycQdVkYYhRA8_wxscBIiMaNcq10gFeFqsKjGDYe1AlQbsvHoXNp1CC8vEUy6zT4btZl_Tjhe8AAAD__w%3D%3D&

vars: "d2 v0.6.8" {
  d2-config: {
    layout-engine: elk
  }
}

OK: {
  foo: {
    a: aaaaaaaaa
    b: bbbbbbbbb {
      d: ddddddddd
    }
  }

  bar: {
    c: ccccccccc
  }

  foo.a -> bar.c
  bar.c -> foo.b.d
}

NG: {
  foo: {
    a: aaaaaaaaa
    b: bbbbbbbbb\nbbbbbbbbb {
      d: ddddddddd
    }
  }

  bar: {
    c: ccccccccc
  }

  foo.a -> bar.c
  bar.c -> foo.b.d
}