实体框架核心，如何为.HasOptional配置可空的外键等效项

public class Order
{
    public int OrderId { get; set; }
    public string OrderNumber { get; set; }
    public int? CustomerId { get; set; } // 可空的外键

    public virtual Customer Customer { get; set; }
}

public class Customer
{
    public int CustomerId { get; set; }
    public string CustomerName { get; set; }

    public virtual ICollection<Order> Orders { get; set; }
}

public class MyDbContext : DbContext
{
    public DbSet<Order> Orders { get; set; }
    public DbSet<Customer> Customers { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>()
            .HasOptional(o => o.Customer) // 配置可空的外键等效项
            .WithMany(c => c.Orders)
            .HasForeignKey(o => o.CustomerId);
    }
}