Nullify the relation at a more general level.
This allows us to avoid hacks like the "return 0 if owner.new_record?" in #count (which this commit removes). Also, the relevant foreign key may actually be present even on a new owner record, in which case we *don't* want a null relation. This logic is encapsulated in the #null_scope? method. We also need to make sure that the CollectionProxy is not 'infected' with the NullRelation module, or else the methods from there will override the definitions in CollectionProxy, leading to incorrect results. Hence the nullify: false option to CollectionAssociation#scope. (This feels a bit nasty but I can't think of a better way.)
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