BUG: truncate has incorrect behavior when index has only one unique v…
…alue (#42365)

- [x] closes #42365
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry
