@Entity
public class Order {
  private Customer customer;
 
  public void setCustomer(Customer customer) {
    this.customer = customer;
  }

  public Customer getCustomer() {
        if (customer == null)
          return Customer.createNullCustomer();
        else
          return this.customer;

  }

 
}

@Entity
public class Customer {
  static Customer createNullCustomer() {
    return new NullCustomer();
  }

  public boolean isNull() {
    return false;
  }
}

public class NullCustomer extends Customer {
  public boolean isNull() {
    return true;
  }
}

@Entity
public class Item{
    private Customer customer;
    private Order order;

    public void doSomething() {
      this.customer = order.getCustomer();
    }
}