How to stablish on delete action but skiping on severed (set to null)

Question

Here are the classes example

public class Parent
{
    public int ParentId { get; set; }
    public string Name { get; set; }
    public Child Child { get; set; }
}

public class Child
{
    public int ChildId { get; set; }
    public string Note { get; set; }
    public int? ParentId { get; set; }
    public Parent Parent { get; set; }
}

here is the Fluent configuration

modelBuilder.Entity<Child>()
            .HasOne(x => x.Parent)
            .WithOne(x => x.Child)
            .OnDelete(DeleteBehavior.Cascade);

and here is the operation.

var p = await _context.Parents
          .Where(x => x.ParentId == id)
          .Include(x => x.Child)
          .FirstOrDefaultAsync();

var p2 = await _context.Parents
          .Where(x => x.ParentId == id2)
          .Include(x => x.Child)
          .FirstOrDefaultAsync();

//Below works just fine, remove the parent and child
_context.Parents.Remove(p);

//In this two is where I want to conserve the child in DB.
p2.Child = null;
//Or
p2.Child = new ();

_context.Update(p2);
_context.SaveChanges();

What I'm looking for is: to delete the dependent side when I delete the Principal, Done. If I set null the Child in the Parent, preserve the Child.

Any clue on this is welcome, thanks.