text: Title: [Discussion]: Enforce values in parser test cases.
  Body: There are currently test cases like [this](https://github.com/flairNLP/fundus/blob/master/tests/resources/parser/test_data/de/Focus.json) which include empty values for attributes. I would consider those as a bad test case since they cover less than they could. To avoid example HTML like this in the future I would suggest adding an assertion to `test_parsing()` asserting `bool(attr)` if attr is not of type `bool`.
  
  Update:
  Points to a valid example JSON again. Thanks to @dobbersc for pointing this out.