TST: Add test for clip-na
Additional test cases for https://github.com/pandas-dev/pandas/pull/16364
when upper and / or lower is nan.

cc @jreback [this line](https://github.com/pandas-dev/pandas/pull/16364/files#diff-03b380f521c43cf003207b0711bac67fR4112) was missing tests I think.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=h1) Report
> Merging [#16369](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/42e2a87f2a8848795238de1259a3daa5612e393d?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16369/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16369      +/-   ##
==========================================
- Coverage    90.4%   90.37%   -0.04%     
==========================================
  Files         161      161              
  Lines       50933    50933              
==========================================
- Hits        46048    46031      -17     
- Misses       4885     4902      +17
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.15% <ø> (-0.02%)` | :arrow_down: |
| #single | `40.21% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.12% <0%> (+0.16%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16369?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/16369?src=pr&el=footer). Last update [42e2a87...b546752](https://codecov.io/gh/pandas-dev/pandas/pull/16369?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 

I think ok to merge this. I will push another PR for clip (to handle int/float things).

going to mark 0.20.2 for consistency.