如何使实体框架核心创建不强制的外键约束

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public int? AuthorId { get; set; } // 注意这里的int?表示可以为null
    public Author Author { get; set; }
}

public class MyDbContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Book>()
            .HasOne(b => b.Author)
            .WithMany(a => a.Books)
            .HasForeignKey(b => b.AuthorId) // 这里没有设置IsRequired
            .OnDelete(DeleteBehavior.SetNull); // 当作者被删除时，将书籍的外键设置为NULL
    }

    public DbSet<Author> Authors { get; set; }
    public DbSet<Book> Books { get; set; }
}