prevent php8-warnings, closes #1770

app/controllers/admission/courseset.php

app/controllers/admission/rule.php

app/controllers/course/forum/index.php

app/controllers/course/room_requests.php

app/controllers/resources/room_request.php

app/controllers/tour.php

app/controllers/userfilter/field.php

app/controllers/userfilter/filter.php

app/views/admission/courseset/configure.php

app/views/admission/courseset/configure_courses.php

app/views/admission/courseset/institutes.php

app/views/course/details/index.php

app/views/course/forum/index/_threads.php

app/views/course/forum/index/index.php

app/views/course/room_requests/request_summary.php

app/views/resources/building/_add_edit_form.php

app/views/userfilter/field/configure.php

...rules/conditionaladmission/ConditionalAdmission.class.php

...les/coursememberadmission/CourseMemberAdmission.class.php

...trictedadmission/ParticipantRestrictedAdmission.class.php