BUG: categorical-with-nas to float64 instead of object
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

This makes us match what we do in Index._find_common_type_compat, after which we'll be able to refactor both into a helper.
great @jbrockmendel i believe you mentioned you can now refactor find_common_dtype, great