[#556] Solve NPE when SurveyedLocale has no geocells set

The `/webapp/testharness?action=deleteSurveyResponses` results in an NPE when there are no geocells for a SurveyedLocale.  This is observed when testing issue #613.
Although I am not aware of the `List` implementation the geoCells object is using, it's normally a safer bet to use `isEmpty()` instead of `size() == 0`. Some implementations (in particular some the Datanucleus implementations, used to fetch data from the datastore in FLOW), use a lazy loading approach, inflating all the items lazily, thus taking up much more time when the first call to `size()` is made. However, if we are sure the instance will just be an `java.util.ArrayList`, it makes no difference probably...