Remove extra newline at the end

[alexeyinkin]

This code

import 'package:yaml_writer/yaml_writer.dart';

void main() {
  const input = [{'a': {'b': 1}}];
  final writer = YAMLWriter();
  print(writer.write(input).replaceAll('\n', '_'));
}

produces:

- _  a: _    b: 1__

Note the double newline at the end while a single newline was expected.