Fix inconsistency with UICollectionView's performBatchUpdates:completion: behaviour

When `UICollectionViews`'s `performBatchUpdates:completion` is called with `nil` for each of the block parameters, the view still animates any changes to the layout (as suggested here http://stackoverflow.com/questions/12999510/uicollectionview-animation-custom-layout). The early return from `PSTCollectionView`'s corresponding method meant that it behaved differently, this change fixes that.