If I understand Frog correctly, you get one point for correctly predicting 0 goals in extra time. If you predicted 0 goals, and there are goals scored, you don't get any points.
If you put down that one or more goals will be scored and the outcome is 0 goals, then you don't get any points.
In other words 0 goals as a prediction could be worth a point if it's correct so it's a worthwhile choice in its own right...
The other part of your question still needs to be addressed, though. At the moment it seems that if you think goals are going to be scored then you might as well put down a high number because there is no disincentive not to. Whatever the number scored, they will all then count.
Think Frog needs to opine.