prevent php8-warnings, refs #1733

app/controllers/admin/statusgroups.php

app/controllers/course/members.php

app/controllers/course/messenger.php

app/controllers/course/statusgroups.php

app/controllers/course/wizard.php

app/controllers/jsupdater.php

app/controllers/messages.php

app/controllers/module/module.php

app/controllers/oer/admin.php

app/controllers/profile.php

app/controllers/shared/contacts.php

app/controllers/studiengaenge/shared_version.php

app/controllers/studiengaenge/studiengangteile.php

app/views/admin/plugin/search.php

app/views/course/dates/index.php

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

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

app/views/course/plus/index.php

app/views/course/statusgroups/_member.php

app/views/course/statusgroups/getgroup.php