Welcome to the Asana Community @Erlend!
Yeah I was experiencing the same problem in our team. I have amended some fields to be single or multi-select drop downs that the team has to fill (for specific things such as was xx or xx done) and if nothing is selected and the task is marked complete then the Asana rule opens the tasks again, adds a comment and assigns it.
Now for text or number fields that is not really possible.
I have found this existing feedback request threat so I will merge your post into that one. Required field to be filled in before allowing a task to be "completed"
Don’t forget to upvote there 