def arabic_to_roman(number=None):
    """Converts an arabic number to a Roman numeral."""
    conv = [[1000, "M"], [900, "CM"], [500, "D"], [400, "CD"],
            [ 100, "C"], [ 90, "XC"], [ 50, "L"], [ 40, "XL"],
            [  10, "X"], [  9, "IX"], [  5, "V"], [  4, "IV"],
            [   1, "I"]]

    if number is None or number == "":
        return ""
    else:
        number = int(number)

    result = ""
    for denom, roman_digit in conv:
        result += roman_digit * (number // denom)
        number %= denom
    return result