fix warnings on PHP 8, fixes #160

exercises/cloze_exercise.php

exercises/lt_exercise.php

exercises/me_exercise.php

exercises/seq_exercise.php

exercises/tb_exercise.php

lib/DummyExercise.php

views/exercises/cloze_exercise/edit.php

views/exercises/lt_exercise/edit.php

views/exercises/seq_exercise/edit.php

views/exercises/tb_exercise/edit.php

views/exercises/tb_exercise/print.php

views/sheets/print_assignment.php