CLN: simplify combine_first
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20972?src=pr&el=h1) Report
> Merging [#20972](https://codecov.io/gh/pandas-dev/pandas/pull/20972?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd4332f4bff135d4119291f66e98f76cc5f9a80e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20972/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20972?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20972      +/-   ##
==========================================
+ Coverage   91.81%   91.81%   +<.01%     
==========================================
  Files         153      153              
  Lines       49481    49474       -7     
==========================================
- Hits        45430    45427       -3     
+ Misses       4051     4047       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <100%> (ø)` | :arrow_up: |
| #single | `41.85% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20972?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20972/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <100%> (+0.09%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20972/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20972?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/20972?src=pr&el=footer). Last update [bd4332f...15e2221](https://codecov.io/gh/pandas-dev/pandas/pull/20972?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
