🐛 Bug: Multi-line reports with empty messages

[johnyob]

Context

When printing multi-line labels with empty messages, the 'bottom line' is missing:

Current Behaviour

    error[E012]: generic type variable escapes its scope
        ┌─ expect_test.ml:2:3
      1 │    let escape = fun f ->
      2 │ ╭    forall (type 'a) ->
      3 │ │      (f : 'a -> 'a)
        │ ╰──
      4 │    ;;

Expected Behaviour

    error[E012]: generic type variable escapes its scope
        ┌─ expect_test.ml:2:3
      1 │    let escape = fun f ->
      2 │ ╭    forall (type 'a) ->
      3 │ │      (f : 'a -> 'a)
        │ ╰───────────────────^
      4 │    ;;