BUG: fix to_latex() when using MultiIndex with NaN in (#14249)
- [x] closes #14249
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This attempt to close #14249 which actually reports two related issues (one in the issue one in the SO link) with to_latex() with MultiIndex when the MultiIndex contains a NaN. The output is now more consistent with what you'd get from to_csv() or to_html().