Entity Framework Core ignore null update - .net-core

Question

My EF models are like so:
public class Base
{
public Guid Id { get; set; }
public string Name { get; set; }
}
public class Foo : Base
{
public IEnumerable<Bar> Bars { get; set; }
}
public class Bar : Base
{
...
}
My intention was to build the API in such a way that if you specify null on an update it would discard that value however it does not appear to work that way. In my repository update code I do the following:
public override IEnumerable<Base> Update(IEnumerable<Base> items)
{
foreach (var item in items.OfType<Foo>())
{
var existingItem = _context.Foos.Find(item.Id);
if (existingItem == null)
{
throw new InvalidOperationException(
$"Can't update item of type `{typeof(Foo)}` as it doesn't exist. ");
}
var entry = _context.Entry(existingItem);
entry.CurrentValues.SetValues(item);
foreach (var property in entry.Properties)
{
var original = property.OriginalValue;
var current = property.CurrentValue;
property.IsModified = original != null && !original.Equals(current);
}
var collection = entry.Collection(nameof(Foo.Bars));
if (collection.CurrentValue == null)
collection.IsModified = false;
}
var rows = _context.SaveChanges();
return Read(items.Select(e => e.Id));
}
Since the Foo.Bars property is actually a Collection/Navigation property there's no OriginalValue property to it, only a CurrentValue and this means I can't discard the value if it is null. Also it seems that setting the collection.IsModified to false has no effect and the Foo.Bars property is set to null regardless of the IsModified state.
Looking for advice on perhaps a better way to handle this or something I'm missing. Thanks.