How does the director in the Builder class Sets a null int to 0?

I'm studying a simple example of Builder Design Pattern.

So my question is, if I try to instantiate my Phone class directly, it wont accept null for the int parameter which is understandable since it is a primitive type.

However when I use the builder and I don't set the value of an Int variable(which will remain null) the director somehow changes its value to 0.
Phone p=new PhoneBuilder().setOs("Android").setRam(2).getPhone();

p.Processor == null at this point. so in getPhone() when the phone constructor is called, the value of processor is supposed to be null I tried to watch the variable with the debugger but I can't find nor understand why it changes from null to 0.

public class Phone {    

    private String os;  
    private int ram; 
    private String processor;   
    private double screenSize;   
    private int battery;    

    public Phone(String os, int ram, String processor, double screenSize, int battery) {
        super();
        this.os = os;
        this.ram = ram;
        this.processor = processor;
        this.screenSize = screenSize;
        this.battery = battery;
    }

}

and here is my builder:

    public class PhoneBuilder {

        private String os; 
        private int ram; 
        private String processor;
        private double screenSize;
        private int battery; 

        public PhoneBuilder setOs(String os) {
            this.os = os;
            return this;
        }

        public PhoneBuilder setRam(int ram) {
            this.ram = ram;
            return this;
        //...

        public Phone getPhone() {
            return new Phone(os, ram, processor, screenSize, battery);
        }
    }