Annotates `indexers/utils.py` functions that don't return anything with `None`
- [ ] does not close any issue, change is too trivial
- [ ] tests are not required for this
- [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
- [ ] nothing new

Related https://github.com/python/mypy/pull/11273
