EF Core HasOptional WithOptionalDependent

Question

I have the following tables in my database:

Product                                 ResourceAssociation
-------                                 ------------------
Name                          /-------> Id
ResourceAssociation_Id ------/         SomeProperty

So Product has a ResourceAssociation_Id, which is a FK to the ResourceAssociation table.

This is a legacy application that had an Entity Framework 6 map that contained this piece of code:

HasOptional(t => t.ResourceAssociation).WithOptionalDependent().WillCascadeOnDelete(true);

I'm looking to port this to EF Core. EF Core doesn't have the HasOptional anymore, it seems. I've tried this:

builder.HasOne(t => t.ResourceAssociation)
    .WithOne().HasForeignKey("ResourceAssociation_Id")
    .IsRequired(false)
    .OnDelete(DeleteBehavior.Cascade);

I've also tried replacing the WithOne with WithMany. But it doesn't work.

When I try to create a new Product, without a ResourceAssociation (because it isn't required), EF Core still tells me my ModelState is invalid because the ResourceAssociation is null.

I can't specify the property in WithOne (e.g. WithOne(x => x.Product)) because ResourceAssociation is a table that will also be used for other tables (for example Brand also has a ResourceAssociation_Id column pointing to that table).

Is this kind of setup even possible in EF Core? Or will I have to add the different navigation properties to my ResourceAssociation class (ie Product, Brand,...)?