In the editor the "answer" is optional. Once the activity is created the user is "forced" to give an answer. Wherein if they place something in the answer box it will be marked as incorrect. Results screen also shows the answer as "undefined".
Issue is reported in: https://h5p.org/node/255731
Test content: https://h5p.org/node/256181
Content created in H5P.org
Solution suggested in