Fix left join turning into outer join
- [x] closes #19607
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=h1) Report
> Merging [#19624](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a214915e241ea15f3d072d54930d0e0c8f42ee10?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19624/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19624      +/-   ##
==========================================
+ Coverage   91.59%   91.62%   +0.02%     
==========================================
  Files         150      150              
  Lines       48795    48798       +3     
==========================================
+ Hits        44696    44711      +15     
+ Misses       4099     4087      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.73% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19624/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.42% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19624/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19624/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `60.6% <0%> (+3.93%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=footer). Last update [a214915...0a068a0](https://codecov.io/gh/pandas-dev/pandas/pull/19624?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @elrubio 

if you'd have a look at the xref issues to see if they are closable with this fix (if they are not, would appreciate a PR for those as well!)

> I think this is the same issue as #17257. maybe also related to #16304. Welcome to have a look / PR to fix. These should be the same.