prevent more php8 warnings, fixes #1679 (again)

app/controllers/admission/courseset.php

app/controllers/admission/restricted_courses.php

app/controllers/studip_controller.php

app/views/admin/user/index.php

app/views/admission/courseset/_institute_choose.php

app/views/admission/institute-select.php

app/views/admission/restricted_courses/_institute_choose.php

app/views/admission/restricted_courses/index.php

app/views/admission/user_list/configure.php

app/views/course/feedback/_add_edit_entry_form.php

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

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

app/views/questionnaire/evaluate.php

app/views/siteinfo/markup/adminList.php

app/views/siteinfo/markup/indicator.php

app/views/siteinfo/markup/rootlist.php

app/views/siteinfo/markup/toplist.php

lib/classes/CoursesetModel.php

lib/classes/MediaProxy.php

lib/classes/MyRealmModel.php