Refined the test: @ParameterizedTest adds no additional insight here

Refined the test: @ParameterizedTest adds no additional insight here

* The parameterized tests only test the difference between an empty and an
  non-empty Optional.
* The actual number is meaningless, because the business logic is mocked
  away anyway!
* Hence, the parameterized version only tests the correctness of our mocking:
  The test could only fail, if we make a mistake in the setup of the
  expectations, not in the code under test!
* Also splitted up some test-cases more logically.