BUG: LatexFormatter.write_result multi-index
Fixed GH issue 14484:

`LatexFormatter.write_result(...)` now does not print blanks if a
higher-order index differs from the previous row.
Also added testcase for this.

- [X] closes #14484
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
