Pivot nans fix
this gets the ball rolling on #18030 as it is very easy to fix, but requires some decisions, mostly for structure and naming

the basic options as I see them:

0 - this PR as is (maybe some name change to this 26 character monster)

1 - Outright changing the `dropna=False` default behavior I thought would be a bad idea as it breaks backwards compatibility, but this solution is quite clunky.

2 - Change `dropna=True` behavior to satisfy the test added here, but again that is not backwards compatible, but seems reasonable.

Also, I've noticed that taking out the line
https://github.com/pandas-dev/pandas/blob/3f8c0c4f002b6bcc69e0e534cb0c502fbab00373/pandas/core/reshape/pivot.py#L177
doesn't break any of the tests, so there is that. (this helps 2)

- [x] closes #18030
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
