Deprecation with PHP 8.4

When used with PHP 8.4 following depracations are shown:

• Jejik\MT940\Parser\AbstractParser::getLine(): Implicitly marking parameter $position as nullable is deprecated, the explicit nullable type must be used instead
• Jejik\MT940\Parser\AbstractParser::getLine(): Implicitly marking parameter $length as nullable is deprecated, the explicit nullable type must be used instead