BUG: to_xml raising for pd.NA
- [x] closes #43903
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

hmm some failures that could be relevant It looks like this was some kind of hiccup. But merging master to be sure thanks @phofl 