BUG: Fix na_rep when using pd.NA in _format_strings (#33950)
- [x] closes #33950
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@k-fillmore thanks for the contribution!

Does this keep the correct `"<NA>" `representation by default? (when not using `na_rep` keyword)

Further, can you add a test for this? Hi, @jorisvandenbossche  Doesn't appear to retain the "" representation.  I've added an additional expression to address this. Working on tests.  Hello @k-fillmore! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-06-05 22:52:41 UTC @jorisvandenbossche added tests. Don't really like the leading whitespace that I had to add. But that's how they're being passed back from GenericArrayFormatter. I tried to track down where they're coming from but didnt have much luck. Any ideas?  Just want to follow up on this. Do I need to add anything else?  https://github.com/pandas-dev/pandas/issues/33950#issuecomment-680406172 noted that this affects NaT too. Can you fix that here or open a second issue? Can you also merge master and re-push. I don't know why CI failed last time. We also need a release note in whatsnew/1.2.0.rst. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Noted, working on NaT here. Will do a merge master and add change logs as requested.  Closing for now. @k-fillmore let us know whenever you'd like to continue and we'll reopen!