Guard against nil ratings.

Even though this is a really small change, I am torn on this and would like someone else to either merge or reject this:

This fixes the calculation of the standard deviation in cases where there is an event feedback without numerical rating.

Originally, it sufficed to enter a feedback comment and the numerical rating was not mandatory. These days there is a validation that makes the rating a required value.

That is why older frab installations may still have records where rating is nil. FrOSCon has them. But if FrOSCon's is the only installation that is affected, I would not want to burden everyone else with the extra loop :confused: