Person: nilify gender field if set to empty
Person: nilify gender field if set to empty

For useful stats, it's important not to have a mixture of nil and "" in the
gender field of Person objects. Use an before_save callback to make sure we
always have nil instead of "".

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