Fix issue with the collections appearing in the list page (#16562)

I thought the collections appearing in the list page problem was solved, but this turned out to be a bug with the ttest. Upon actual inspection the lists remained.

To fix this without changing all sorts of crap that depended upon collections api contract (we futzed with the output but the futzing of the output is transparent to the frontend because of the entity thing they got going on), needed a way to elicit null response from collections endpoint. shoved one in. then renamed it because the first name, "dummy", makes it all sound kind of jank, so 'no-model' for explicitly asking for no model without handing in an empty set, which turns into the set of all models.

This is an 80% unjankening. Full unjankening would entail whacking this special case.