Guard against nil ratings.
Guard against nil ratings.

In the beginning, entering a numerical rating was not mandatory.
That is why older frab installations may still have records where
the rating is nil. This changes filters out those records when
calculating the standard deviation.

henryk/frab,fossgis/frab,stapelberg/frab,fossgis/frab,henryk/frab,cketti/frab,cketti/frab,afics/frab,cketti/frab,devfest-berlin/frab,stapelberg/frab,devfest-berlin/frab,afics/frab,henryk/frab,afics/frab,fossgis/frab,gregmolnar/frab,fossgis/frab,devfest-berlin/frab,stapelberg/frab,gregmolnar/frab