BUG: fix #60128 BUG: Series.combine_first loss of precision
- Issue: There was int64->float64->int64 conversions
- Fix: Carry out the operation without passing through float64

- [X] closes #60128
- [ ] [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
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

It seems this needs more general approach, and it would not suitable for a newbie like me. So, I'd like to just close this PR.