ChangeTracker shouldn't throw on conceptual nulls if one of the fk properties is nullable

[AndriySvyryd]

public class Product
{
    public int Id { get; set; }
    public int PartitionId { get; set; }

    public int? CategoryId { get; set; }
    public Category Category { get; set; }
}

public class Category
{
    public int Id { get; set; }
    public int PartitionId { get; set; }

    public ICollection<Product> Products { get; set; }
}

public class StoreContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<Category> Categories { get; set; }

    public StoreContext(DbContextOptions options)
        : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Product>()
            .HasKey(p => new { p.Id, p.PartitionId });
        modelBuilder.Entity<Category>()
            .HasKey(c => new { c.Id, c.PartitionId });

        modelBuilder.Entity<Product>().HasOne(p => p.Category)
            .WithMany(c => c.Products)
            .HasForeignKey(p => new { p.CategoryId, p.PartitionId });
    }
}

context.Products.Add(new Product { Id = 1, PartitionId = 1, Category = new Category { Id = 2, PartitionId = 1 } });
context.SaveChanges();

context.Products.Include(p => p.Category).Single().Category = null;
context.SaveChanges();

Throws:
The association between entity types 'Product' and 'Category' has been severed but the foreign key for this relationship cannot be set to null. If the dependent entity should be deleted, then setup the relationship to use cascade deletes.