Summary:
Currently, if you use `IGListSectionController` without a subclass, `UICollectionView` will crash when requesting the cell. That's because `[IGListSectionController numberOfItems]` defaults to 1, but returns no cell. There's a few issues with that:
1. It can be tough to debug, because the crash in within `UICollectionView`. We don't know the dataSource or object type responsible.
2. The crash only happens if the user scrolls by the missing cell, which can make it hard to repro.
3. If we don't know how to render a single section, we might not want to crash the entire app. Passing a plain `IGListSectionController` kind of feels like the dataSource is ok with just rendering nothing. It's not clear at all that it will crash.
Options:
1. Crash immediately when a plain `IGListSectionController` is passed to `IGListAdapter`, so we get a clear callstack and API feedback. If the dataSource doesn't want to crash, it must return `IGEmptySectionController` that has 0 `numberOfItems`.
2. Don't crash, but log a `IGFailAssert()`. If the dataSource wants to throw on a missing object-controller match, they can can, but it's not the IGListKit default.
I'm leaning on #2 for a few reasons. It's really not obvious that passing a plain `IGListSectionController` would crash and no one will know that `IGEmptySectionController` exists. We could have a linter, but that only works within Meta.
Reviewed By: DimaVartanian
Differential Revision: D52087286
fbshipit-source-id: 8b8754d56e66c0c2b00f8df3b9671a6fc2287aea
