EF Core Property B should be NOT ISNULL(A)

Question

I am using EF Core 6 and have the following question.

In my EF model "MyModel", I have a property.

public byte[] Thumbnail { get; set; }

These thumbnails can have some MB of data. So, I don't load them at the beginning. I now want an additional EF property in "MyModel".

// Should be sql equivalent NOT ISNULL(Thumbnail)
public bool HasThumbnail { get; set; }

That's needed because

// means not loaded yet OR no data at all
if(myModel.Thumbnail == null)
{
}

I know, that I can achieve this by using

context.MyModel.Select(...)

and map my HasThumbnail property in code.

But is there a way to tell the EF ModelBuilder this behaviour so that it works on the fly?