Add an option to treat unfilled drop areas as errors

[apienk]

Could you please add an option to treat unfilled drop areas as errors - that is, deduct a point or mark red, depending on other settings. For certain activities it doesn't make sense for some of the drop areas to be left empty.

Example: Long division https://pistacja.tv/film/mat00080-dzielenie-pisemne-przez-liczby-jednocyfrowe-wprowadzenie?exercise=dzielenie-pisemne-przez-liczby-jednocyfrowe---klocki