Corrects firstItemCount for empty result sets
#6808 #6809 - adding missing `@group` annotations for newly introduced test cases
#6808 #6809 - avoid multiple internal calls to methods when values are already cached in `$pages`
Merge branch 'hotfix/#6809-#6809-empty-zend-paginator-first-page-starts-with-1' into develop

Close #6808
Close #6809