For the purposes my question, it would be a complication, but resolvable by using a unique identifier. Perhaps I should have limited my speculation to databases that are in at least first normal form.

I'm not sure how the 1NF limitation helps the question. One-to-many relations can certainly exist in 1NF. If the question is amended to specify that the table that represents the nullable column must have its primary key also be its foreign key to the main table, then I agree that the answer is "yes".

I thought 1NF was a solution to a follow-on problem with handling repeating groups, that, on reflection, I see does not exist.