BUG: Index.str.partition not nan-safe (#23558)
- [x] closes #23558 and 
closes #23677
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Hello @meiermark! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/test_strings.py`](https://github.com/meiermark/pandas/blob/25e3cc46537a5f3f9ef6fc265d0847c9cf422177/pandas/tests/test_strings.py) !



##### Comment last updated on November 18, 2018 at 01:05 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23618?src=pr&el=h1) Report
> Merging [#23618](https://codecov.io/gh/pandas-dev/pandas/pull/23618?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1250500cfe18f60cfb8a4867c82d14467bbee7ad?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23618/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23618?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23618   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51408    51408           
=======================================
  Hits        47416    47416           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |


------

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 cc @h-vetinari if you want to have a look. @jreback @meiermark 

Not sure if this is closed intentionally or permanently. It's a valid proposal to change the output of `to_object_array_tuples`, but primarily, the problem should be tackled through the MultiIndex constructor, which is the actual problem for `.str.partition`, see #23578. I've got a solution already as part of #23167, but this is currently blocked by / waiting for #23582. Sorry for the confusion. I destroyed my branch and needed to restore it. Unfortunately, the pull request was closed by this. @jreback The whatsnew info was added with the last commit. @meiermark 
This should very likely also solve #23677. Could you have a look, and maybe adapt the tests accordingly as well? @h-vetinari I extended some tests to check #23677. Your assumption was right, it's also resolved.
@jreback can you have a look again? The current state should include the required changes. does this close both issues at the top of the PR? can you annotate a test where they are covered. also add to the whatsnew both issue numbers. @jreback Yes, this PR closes both issues. Added comments to the tests (links to the corresponding issues) and a new whatsnew message. thanks @meiermark keep em coming!