BUG: Respect na_rep in DataFrame.to_latex() when used with formatters (#9046)
- [x] closes #9046
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A very old issue so low priority! 

I'd be tempted to remove the shortcut in `_format_strings()` since I'd guess in most cases it doesn't really save much time. 