Check button doesn't work as expected when no answer is selected

[apienk]

Steps to reproduce:

1. Create a Multiple Choice question.
2. Turn ON the display of Check button.
3. Turn ON the display of Show Solution button (optional but aggravates the issue).
4. Turn OFF the display of Retry button (optional but aggravates the issue).
5. Save and open the question in test mode.
6. Click Check button (do not mark any answers before).
7. Click Show Solution button.

Expected result:

1. When the Check button is clicked, the correct answer is marked.
2. If not, the correct answer is marked after the Show Solution button is clicked.

Observed result:

1. When the Check button is clicked, it just makes all answers inactive - the correct answer is NOT marked. The Show Solution button is shown instead.
2. When the Show Solution button is clicked, the correct answer is NOT marked either. Instead a note saying "Select an answer before showing solution" is displayed.
3. There's no way to see the correct answer.