Handle blank lines in `Yaml.Text()`

[dougbu]

• see https://github.com/aaubry/YamlDotNet/issues/886
• transform blank lines to string.Empty
• remove all trailing whitespace from lines
  • if trailing whitespace is significant in a future test, YAML files can be added
• throw new ArgumentException(...) if a line is insufficiently indented
  • should be easier to fix test than with previous ArgumentOutOfRangeException for this case
• add tests

[aaubry]

This feature has been released in version 15.1.0.