SW: do not count a sole critic rating on the stddev

the only critic for a film should not count to the postive stddev,
as being the only one, defines the avg (=0), which improves the stddev
significantly, without any abberation.
do count only differences to averages if there is an average.
(i.e. affected: Boris Nelepo, Ronald Rovers, without forcing them into >2.5)
gemko reviews