BUG: Fix Series.get failure on missing NaN (#8569)
Handle obscure case where `.get(np.nan)` wouldn't behave as expected.  Fortunately the only change needed is within the unlikely branch, and so performance of get shouldn't be affected at all in the ordinary case.

[X] fixes #8569
[X] tests added / passed
[X] passes git diff upstream/master --name-only -- '*.py' | flake8 --diff
[X] whatsnew entry