Check the Ranking Document you linked to. In a one-to-one relationship the reference is only stored in one Collection and an index is used to access the back-relation. Fauna chooses which a collection to store the reference by alphabetical order of the title of the Collection… not the greatest IMO but it is consistent.
If you want to leave it blank then leave it blank and no connection will be made. You should remove the exclamation that marks the field as required.