Bug fix one element series truncate
- [Y] closes #35544 
- [Y] tests added / passed
- [Y] passes `black pandas`
- [Y] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [Y] whatsnew entry

Fix when trying to truncate a one-element series within a correct date range returned an empty series instead of the originally passed-in series. 