Fix 40420:  Interpret NaN in clip() as no bound.
- [x] closes #40420
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

@dsaxton, I have openend this draft PR to fix #40420. Just to be clear: is the behavior proposed in this PR the expected one? 
I still need to do some cleanup on the proposed code, but I wanted to make sure that the behavior we are targetting is correct. > @dsaxton, I have openend this draft PR to fix #40420. Just to be clear: is the behavior proposed in this PR the expected one?
> I still need to do some cleanup on the proposed code, but I wanted to make sure that the behavior we are targetting is correct.

Yes, I believe according to @jreback and this issue we want to treat any "nullish" value as no bound: https://github.com/pandas-dev/pandas/issues/17276 I think non-green is not related here. thanks @DriesSchaumont 