SimpleSectionedModelCollection should ensure any .loaded event results in loaded state

[danielrhammond]

Basically https://github.com/dropbox/pilot/blob/master/Core/Source/MVVM/SimpleModelCollection.swift#L78 needs to add an empty section [.loaded([])] if there are no sections at all since flattenedState treats [] as .notLoaded

Which leads to the surprising interaction where a consumer sends an empty load that is interpreted as content never having loaded.