BUG: df.nsmallest get wrong results when NaN in the sorting column
- [x] closes #46589
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Should this pr be recorded in 1.4.3 whatsnew? I see your 1.4.3 tag (instead of 1.5) on this issue. @simonjayhawkins  Yes, this should move to 1.4.3 thanks @GYHHAHA  @meeseeksdev backport 1.4.x Could not push to auto-backport-of-pr-46656-on-1.4.x due to error, aborting. Could not push to auto-backport-of-pr-46656-on-1.4.x due to error, aborting. @meeseeksdev backport 1.4.x Could not push to auto-backport-of-pr-46656-on-1.4.x due to error, aborting.