BUG-23428 Series.get fills in defaults correctly for multiple indices
- [X] closes #23428
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR makes the default parameter in `Series.get` behave as expected with multiple indices. Some of the testing classes had to be altered to make them work with receiving `None` (the default value for the `default` parameter in `Series.get`).