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.