how to update all the fields with specific value in a list of object

I have a list of an object, I want to update all the fields that are null to string.empty.

here is the code:

public class Class1
{
    public string P1 { get; set; }
    public string P2 { get; set; }
    public string P3 { get; set; }
}

I want to have a code which goes and find all the null values in all the fields and change the value to string.empty

     static void Main(string[] args)
    {

        var list= new List<Class1>();
        var class1 = new Class1 {P1 = "P1-1", P2 = "P2-1", P3="null"};
        list.Add(class1);
        var class2 = new Class1 { P1 = "P1-2", P2 = "P2-2", P3 = "null" };
        list.Add(class2);

    }

so I would need the class1.P3 and class2.P3 to be found and their value to be replaced.

Thanks