fix(mapping): properly handle nil index paths

ensure that the index path mapper returns a nil index path if a nil index path is passed to one of the mapping methods `-dynamicIndexPathForStaticIndexPath:` or `-staticIndexPathForDynamicIndexPath:`
Merge pull request #10 from Hotel-Reservation-Service/feature/fix-index-path-mapper-nil-index-path

fix(mapping): properly handle nil index paths