GROOVY-5865: DefaultGroovyMethods.getAt(List, Range) delegates now to getAt(List, EmptyRange) in case of EmptyRange.

This fixes issue with inconsistent behavior of getAt(List, Collection) where element of collection is EmptyRange. For details please see the JIRA ticket.
GROOVY-5865: getAt(EmptyRange) not called when passing an EmptyRange to getAt(Collection) (minor refactor)