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

looks fine can you add a whatsnew note in 1.4 bug fixes, indexing section @jreback I think this PR should be good as well since the errors above seem transient and unrelated to any of the code changes here (and I'm seeing similar errors in other PRs) thanks @neelmraman 