Fix fatal error that occurred when viewing quiz attempts with deleted questions.

[ideadude]

Description

Viewing quiz attempts when questions were deleted would throw a fatal error:

PHP Fatal error: Uncaught Error: Call to a member function get() on null in ...\wp-content\plugins\lifterlms\templates\quiz\results-attempt-questions-list.php

How has this been tested?

To recreate the issue:

• Create a quiz with 2 questions.
• Complete the quiz from the frontend.
• Delete 1 of the questions.
• Go to LifterLMS > Reports > Quizzes
• Click on the quiz, then Click on Attempts.

Types of changes

This issue was introduced by accident in the work to add the quiz resume feature. Some code was incorrectly updated to get data from the $question variable instead of the $attempt_question variable.

Note: Since $attempt_question->get('question_type') is always null, we instead add type--deleted as a css class on the li element instead of trying to find the type.

Checklist:

• This PR requires and contains at least one changelog file.
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.