BUG: fixed json_normalize for subrecords with NoneTypes (#20030)
TST: additional coverage for the test cases 

DOC: added changes to whatsnew/v0.23.0.txt

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20030
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
