Add default setting values if undefined

[owenherbert]

Linking issue #6

It looks like this fixes the issue. I have retested unit tests and grade setting saving from the user interface.

Before fix:

There were 83 errors:

1) core\grade_category_test::test_grade_category
Undefined property: stdClass::$hidecorrect

/var/www/site/mod/quiz/accessrule/hidecorrect/rule.php:217
/var/www/site/mod/quiz/accessmanager.php:156
/var/www/site/mod/quiz/lib.php:1195
/var/www/site/mod/quiz/lib.php:106
/var/www/site/course/modlib.php:137
/var/www/site/lib/testing/generator/module_generator.php:278
/var/www/site/mod/quiz/tests/generator/lib.php:100
/var/www/site/lib/testing/generator/data_generator.php:503
/var/www/site/lib/grade/tests/fixtures/lib.php:104
/var/www/site/lib/grade/tests/fixtures/lib.php:73
/var/www/site/lib/phpunit/classes/advanced_testcase.php:80
phpvfscomposer:///var/www/site/vendor/phpunit/phpunit/phpunit:97

After fix:

root@c6158c56e24c:/var/www/moodle-41-site# vendor/bin/phpunit --testsuite core_grades_testsuite
Moodle 4.1.10+ (Build: 20240510), ff50214601f73056ed3f2a2acdf5abcdbb291f68
Php: 7.4.33, mysqli: 8.0.34, OS: Linux 6.5.0-28-generic x86_64
PHPUnit 9.5.28 by Sebastian Bergmann and contributors.

...............................................................  63 / 200 ( 31%)
............................................................... 126 / 200 ( 63%)
............................................................... 189 / 200 ( 94%)
...........                                                     200 / 200 (100%)

Time: 05:35.272, Memory: 119.00 MB

OK (200 tests, 2315 assertions)