BUG: Series.is_unique has extra output if contains objects with __ne__ defined


- [x] closes #20661
- [x] tests added / passed
  - series/test/series/test_analytics.py:test_is_unique_class_ne
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Simple change from `_checknan` to `checknull` in pandas/_libs/hashtable_class_helper.pxi.in

Checked performance, and there is no difference.