ENH: Json fill_value for missing fields
- [x] closes #16918
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A new argument `fill_value` is added to allow users to supply default values for missing fields for each column. This enhancement also addresses the undesirable type conversion from `int` to `float` due to missing fields as raised in #16918