BUG: Fix na_position type in IndexEngine
- [x] closes #58924 
- [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  
  (I modified an existing test to cover the problematic case)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).  
  (Stubs check fails locally for seemingly unrelated reasons. Passed on GH Actions.)
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.  
  (**Not applicable**)
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Perhaps I should not have rebased. The changes suggested above by @mroeschke are the only changes, I promise. Thanks @avm19 