How to allow a string to begin with a "." decimal point, using a value converter

The code below does not allow the entry in which it is binded to allow the field to begin with a decimal point. The Entry has a placeholder of 0.
Please help. Thanks

public class DecimalCommaDisplayConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value == null || !(value is decimal)) throw new ArgumentException("This converter may only be used on values of type decimal.");

        var decimalValue = (decimal)value;

        //In the UI represent 0 as empty string.
        if (decimalValue == default)
        {
            return string.Empty;
        }

        //Format the string with maximum of 4 decimal places and commas as appropriate.
        // Examples: 0.23, 1.2, 1,234.5678
        return decimalValue.ToString("#,0.####");
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value != null && !(value is string)) throw new ArgumentException("This converter may only be used on values of type string.");

        string stringValue = (string)value;

        //In the UI, 0 is represented as an empty string.
        if (string.IsNullOrEmpty(stringValue))
        {
            return 0.0m;
        }

        if(decimal.TryParse((stringValue).Replace(",", ""), out decimal result))
        {
            return result;
        }

        //If the value isn't convertable, then just return it and let the TextChanged handler convert it back.
        return stringValue;
    }