PERF: Fix performance regression with Series statistical ops (#25952)
- [x] closes #25952
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

#25743 introduced a performance regression in `Series` `MultiIndex` statistical operations.

This happened due to the additional required check for Series groupby operations that already existed for `DataFrame`.  By ensuring that the check needs to be run, this regression can be avoided also potentially slightly speeding up similar `DataFrame` operations.