prevent php8-warning, closes #1750

app/controllers/course/basicdata.php

app/controllers/course/files.php

app/controllers/course/studygroup.php

app/controllers/file.php

app/controllers/files.php

app/controllers/institute/files.php

app/controllers/messages.php

app/controllers/resources/booking.php

app/controllers/resources/room_request.php

app/views/admin/sem_classes/details.php

app/views/course/overview/index.php

app/views/course/statusgroups/index.php

app/views/file/add_files_window.php

app/views/file/edit_license.php

app/views/resources/booking/_add_edit_form.php

app/views/resources/resource/permissions.php

app/views/resources/room_request/planning.php

lib/classes/Icon.class.php

lib/classes/MultiPersonSearch.class.php

lib/classes/Seminar.class.php