Show parallax header even if there are 0 sections

Should resolve #22 . It was much easier than I though. Apple docs state:

> Note: Although standard index paths support multiple levels, the collection view’s cells only supports index paths that are 2-levels deep with “section” and “item” parameters, much like the index paths for the UITableView class. Supplementary views and decoration views can have more complex index paths if necessary. Elements whose index paths are > 1 is interpreted to correspond to the section designated by the first index in the path. Traditionally, only a second index is necessary, but supplementary and decoration views are not restricted to just two. Keep this in mind when designing your data source.

So using index path of size 1 doesn't seem to require any sections to be present. The old implementation caused inconsistent behavior when using NSFetchedResultsController. Having no items and `nil` as section key showed the parallax header. While having no items and using anything but `nil` as section key didn't show the parallax header.