Once you add a resolution message in any idea for the first time, you can no longer leave it blank if you're editing/updating it, thus the input becomes required (field) just like other required fields (screenshot).
I. Edit any idea which doesn't have any resolution message added yet (example).
II. Add any resolution message.
III. Edit the same idea again and try to remove the resolution message (leaving it blank) and it won't let you (as the field becomes required).