BUG: GH12558 where nulls contributed to normalized value_counts
- [x] closes #12558
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

@nickeubank tests passed. Does this look OK?
 done
 Looks great!
 thanks @kawochen @nickeubank 
