Bug: Extra empty line in sequences containing comments 

Hi !
When writing a sequence of instructions with interleaved comments, `ocamlformat` with profile `conventional` adds a extra line before the comment:
```ocaml
let () =
  print_string "Hello, world!";
  (* This is a comment *)
  print_newline ()
```
is formatted to
```ocaml
let () =
  print_string "Hello, world!";

  (* This is a comment *)
  print_newline ()
```
