Empty Section View

[jessesquires]

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.