Test in scripts/validate_docstrings.py that the short summary is always one line long
- [x] closes #22615
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The previous test to check if the summary property should be empty was based on the non-existence of the extended summary which doesn't seem to make sense. The test case I added failed before changing the script.
Hello @Moisan! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`scripts/tests/test_validate_docstrings.py`](https://github.com/Moisan/pandas/blob/108d092a50f7a374ef1f914e6e93fbe239c439f8/scripts/tests/test_validate_docstrings.py) !

 - There are no PEP8 issues in the file [`scripts/validate_docstrings.py`](https://github.com/Moisan/pandas/blob/108d092a50f7a374ef1f914e6e93fbe239c439f8/scripts/validate_docstrings.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22617?src=pr&el=h1) Report
> Merging [#22617](https://codecov.io/gh/pandas-dev/pandas/pull/22617?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/46abe186d21f2bab3c5df53c906b6a62d07e922b?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/22617/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22617?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22617   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50782    50782           
=======================================
  Hits        46744    46744           
  Misses       4038     4038
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (ø)` | :arrow_up: |
| #single | `42.26% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22617?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/22617?src=pr&el=footer). Last update [46abe18...8e08dd6](https://codecov.io/gh/pandas-dev/pandas/pull/22617?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @datapythonista pls merge if ok thanks @Moisan 