Python: Improve test coverage in 5 places for production code

The production code is mostly covered by the tests. Sometimes, `__repr__` or if conditions are not covered though. That would be nice!

To close the issue:
- [ ] cover 5 additional spots in the tests (167 lines to choose from)
- [ ] create a pull request with your changes and get it merged

What will help:

- Set up for [Development](https://icalendar.readthedocs.io/en/latest/contribute/development.html).
- [Run tests](https://icalendar.readthedocs.io/en/latest/contribute/development.html#run-tests).
- After running the tests, you will have a coverage report as an HTML file at `htmlcov/index.html` as well as console output.
- Create a pull request and we will see the improvements.

Set up the repository using the Development guide, run the test suite, and inspect the console coverage output or htmlcov/index.html. Choose five uncovered production-code spots, especially the mentioned __repr__ or conditional branches, and add tests for them. Done means five additional spots are covered and the changes are submitted in a pull request.

Written by the indexing model from the issue text.