Fixed: Using keyboard navigation in empty CPCollectionView threw exception.
lint
Merge pull request #1842 from stewa/empty-collectionview-keyboard-navigation-problem

Fixed: using keyboard navigation in empty CPCollectionView threw exception

Previously, if an empty CPCollectionView was first responder and keyboard navigation was used, a CPInvalidArgumentException (Range {-1, 1} is out of bounds) was thrown.

This commit checks for an empty collection, eliminating the exception.