BUG: Fix json_normalize throwing AttributeError (#21608)
This PR fixes the bug in `json_normalize` that causes it to throw `AttributeError` when flattening an array of values.
- [ ] closes #21608 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
