handles exchange command (x) with empty hold space differently from GNU (missing  newline)


  Rust (incorrect - missing newline)
```
  $ printf "a\nb\nc" | cargo run -p sed -- '2x'
  a
  c
  # Exit code: 0

```

  GNU (correct)
```
  $ printf "a\nb\nc" | /usr/bin/sed '2x'
  a

  c
  # Exit code: 0
```