[Prediction Challenge] Alternative UX for prediction result when there is 0 completion

When prediction result is set, the challenge will expire.

Therefore, theoretically, setting of prediction result when there is 0 completion should not be allowed. But, the UI still shows the checkbox but does not allow selection. 

Suggestion:

• Consider hiding the option to check the box, and have a note to remind Admins that result can only be set when there is >0 completion