BUG: assert_attr_equal with numpy nat or pd.NA
- [ ] closes #xxxx
- [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

This started off with editing na_value_for_dtype so that it could be re-used in #39446, ended up snowballing into assert_attr_equal.  If requested, I'll split off the bugfix into its own PR.