Leaving T/F blank results in report always listing answer as true

The end of quiz report will always show the correct answer as "True" when the question is left blank. This is because this line does not function correctly if the user answer is null:

$correct_answer = $answer['is_correct'] ? $answer['answer'] : !$answer['answer'];

Since the actual correct answer is already available, I am replacing it with this:

$correct_answer = $this->question->correct_answer;

This is a 4.x issue only as this area of code has changed in 5.x.