Fix side effects of question duplication bug patch

[ngandrass]

I did some testing and 59cda8c seems to introduce side effects that make mod/quiz/tests/backup/repeated_restore_test.php fail.

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Before 59cda8c:

/usr/share/nginx/www/moodle $ php vendor/bin/phpunit --color mod/quiz/tests/backup/repeated_restore_test.php
Moodle 4.5.3 (Build: 20250317), d069cb93378d39ee00ce5f30cc520aaee4ae496c
Php: 8.3.19, pgsql: 14.17, OS: Linux 6.13.8-arch1-1 x86_64
PHPUnit 9.6.18 by Sebastian Bergmann and contributors.

.....SSSS.SS..SS...S.S..................S....................S.  63 / 115 ( 54%)
.....SSSS.SS..SS...S.S....S..S.SS.S.....S..S........            115 / 115 (100%)

Time: 00:56.384, Memory: 72.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 115, Assertions: 232, Skipped: 29.

After 59cda8c:

/usr/share/nginx/www/moodle $ php vendor/bin/phpunit --color mod/quiz/tests/backup/repeated_restore_test.php
Moodle 4.5.3 (Build: 20250317), d069cb93378d39ee00ce5f30cc520aaee4ae496c
Php: 8.3.19, pgsql: 14.17, OS: Linux 6.13.8-arch1-1 x86_64
PHPUnit 9.6.18 by Sebastian Bergmann and contributors.

.....SSSS.SS..SS...S.S..................S....................S.  63 / 115 ( 54%)
..EEESSSSESSEESSEEESESEEEESEESESSESEEEEESEESEEEEEEEE            115 / 115 (100%)

Time: 00:43.563, Memory: 70.00 MB

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The cause was:

Undefined array key "coderunner_testcases"

/usr/share/nginx/www/moodle/question/type/coderunner/backup/moodle2/restore_qtype_coderunner_plugin.class.php:154

Therefore a check for this is added to the respective function.

[trampgeek]

Thanks Niels.