TST: NaN in MultiIndex should not become a string (#7031)
This verifies that NaNs in multiindexes don't become strings under outer joins or during arithmetic.  This seems to have been fixed between 0.14.1 and 0.15.0.

Too minor and internal for a whatsnew update.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=h1) Report
> Merging [#16625](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/10c17d4896799cf1e8f313025d1071e5abbb65f9?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16625/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16625      +/-   ##
==========================================
- Coverage   90.96%   90.93%   -0.03%     
==========================================
  Files         161      161              
  Lines       49263    49263              
==========================================
- Hits        44810    44798      -12     
- Misses       4453     4465      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.22% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=footer). Last update [10c17d4...9fe83d7](https://codecov.io/gh/pandas-dev/pandas/pull/16625?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!