T&A 45201: Fixes undefined key when copying certain tests with multiple choice questions with feedback

[matheuszych]

This change will probably fix the issue mentioned here: https://mantis.ilias.de/view.php?id=45201

The cause seems to be dangling data inside the qpl_fb_specific table with no associated question answers.

Pretty much the same issue was fixed in this PR but only for Single Choice questions. https://github.com/ILIAS-eLearning/ILIAS/commit/7f7e75874e602e9bd8756cc2c4ce42688826a4e6

From what is seems this should also be cherry picked to ILIAS 10, since it is also missing there.

This should be a valid fix, because this was the original intention here. In PHP 7 a undefined key would throw a notice but still return null. Since PHP 8 it throws a warning instead.