public void GenerateReport(int ReportId, [Optional] string saveToFolderPath)

// Use the C# syntax for this, rather than specifying the attribute explicitly
public void GenerateReport(int ReportId, string saveToFolderPath = null)
{
    saveToFolderPath == saveToFolderPath ?? Directory.GetCurrentDirectory();
    ...
}

public void GenerateReport(int ReportId)
{
    GenerateReport(ReportId, Environment.CurrentDirectory);
}
public void GenerateReport(int ReportId, string saveToFolderPath)
{
}