_get_new_index always returns index in PeriodIndexResampler
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Cleans up a method that returned different types depending on the length of the index

was this meant to be a PR?
 Yes! Split this out of the other given it wasn't directly impacting the issue. 
Does the point make sense?
 can you add some tests for validation, or you can just roll this into the other issue
 OK I will roll in. I know you at times prefer things split out where it doesn't directly impact; easier for me to roll it in though
 Rolling into #12874
