Как игнорировать правило FluentValidation, если объект имеет значение null?

// no need to use IgnorePatternWhiteSpace:
Console.WriteLine(TelNumberValidation("(909)122-5879"));        //true
Console.WriteLine(TelNumberValidation("900 100 1649"));         //true
Console.WriteLine(TelNumberValidation("9001001649"));           //true
Console.WriteLine(TelNumberValidation("900.100.1649"));         //true          
Console.WriteLine(TelNumberValidation("900 100 5478"));         //true
Console.WriteLine();
 
// not using Named Captures
Console.WriteLine(TelNumberValidation("(909)122-5879", 1));     //true
Console.WriteLine(TelNumberValidation("900 100 1649", 1));      //true
Console.WriteLine(TelNumberValidation("9001001649", 1));        //true
Console.WriteLine(TelNumberValidation("900.100.1649", 1));      //true
Console.WriteLine(TelNumberValidation("900 100 5478", 1));      //true
Console.WriteLine();
 
// if ExplicitCapture option is used
Console.WriteLine(TelNumberValidation("(909)122-5879", 2));     //true
Console.WriteLine(TelNumberValidation("900 100 1649", 2));      //true
Console.WriteLine(TelNumberValidation("9001001649", 2));        //false
Console.WriteLine(TelNumberValidation("900.100.1649", 2));      //true
Console.WriteLine(TelNumberValidation("900 100 5478", 2));      //true
Console.WriteLine();
 
// 11/10-digit numbers with and without a prefix (Russian standard)
Console.WriteLine(TelNumberValidation("(909)122-5879", 3));     //true
Console.WriteLine(TelNumberValidation("8(900)100-1649", 3));    //true
Console.WriteLine(TelNumberValidation("8(900)100-167", 3));     //false
Console.WriteLine(TelNumberValidation("+7(900)100-1649", 3));   //true
Console.WriteLine(TelNumberValidation("88(900)100-1649", 3));   //false
Console.WriteLine(TelNumberValidation("900 100 5478", 3));      //false
Console.WriteLine(TelNumberValidation("900 100 444", 3));       //false
Console.WriteLine();
 
 
static bool TelNumberValidation(string snum, int vp = 0)
{
    string pattern = vp switch
    {
        1 => @"^(?:\(?)(\d{3})(?:[\).\s]?)(\d{3})(?:[-\.\s]?)(\d{4})(?!\d)",
        2 => @"^\(?(?<AreaCode>\d{3})[\).\s]?(?<Prefix>\d{3})[-\.\s](?<Suffix>\d{4})(?!\d)",
        3 => @"^((8|\+7)?(\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}$",
        _ => @"^(?:\(?)(?<AreaCode>\d{3})(?:[\).\s]?)(?<Prefix>\d{3})(?:[-\.\s]?)(?<Suffix>\d{4})(?!\d)"
    };
 
    Regex rg = new(pattern);
    MatchCollection m = rg.Matches(snum);
    return m.Count > 0;
}
Console.Read();