message: Don't handle lines with only whitespace
diff: @@ -111,7 +111,7 @@ pub(crate) fn readln(shell: &mut Shell) -> Option<String> {
  match line {
  Ok(line) => {
  - if !line.is_empty() {
  + if line.bytes().any(|c| !c.is_ascii_whitespace()) {
  shell.flags |= flags::UNTERMINATED;
  }
  Some(line)
  