BUG: DataFrame.diff with dt64 and NaTs
- [x] closes #32441
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

There's a related bug in algos.diff, but the elegant solution goes through the cython code, so im doing that in a separate branch.