ENH: recognize Decimal("NaN") in pd.isna
- [x] closes #23530
- [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

Discussion in #23530 seems ambivalent on whether this is desirable, and I don't have a strong opinion on it in general.  BUT tm.assert_foo_equal is incorrect with Decimal("NaN") ATM and id like to see that fixed.

xref #32206