Support empty sections (headers/footers only) + Empty Section View?

Currently, sections that are empty (no cells) are removed. Should we support allowing empty sections to stay in the collection? The use case would be showing a header/footer view. This is arguably not good UI/UX. Up for debate.

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We support providing an "Empty View" to display for the entire collection when it is empty. (e.g.,`ContentUnavailableView`)

It might be cool to offer this on a per-section basis.

Implementation will be very different than for the whole collection.

We'll need to use a `CellViewModel` as a placeholder to keep the section in the collection. Then pretend it's empty, when it only has the placeholder "empty state" cell.