Return `null` instead of empty string when determining classes from enum

[RiRiSharp]

Case in point:

    internal static string ToDropdownBootstrapClass(this BsDirection direction)
    {
        return direction switch
        {
            BsDirection.Start => "",
            BsDirection.End => "dropdown-menu-end",
            BsDirection.Down or BsDirection.Up => throw new ArgumentOutOfRangeException(
                nameof(direction),
                direction,
                $"Only {nameof(BsDirection.Start)} or {nameof(BsDirection.End)} are valid for dropdown menu alignment"
            ),
            _ => throw new ArgumentOutOfRangeException(nameof(direction), direction, null),
        };
    }