Initialising ReducerAggregator if (when) state is null.

This is an attempt at overcoming the problem outlined here:
https://groups.google.com/d/topic/storm-user/3kQU_8FO1xg/discussion

src/jvm/storm/trident/state/ReducerValueUpdater.java

@@ -15,10 +15,10 @@ public ReducerValueUpdater(ReducerAggregator agg, List<TridentTuple> tuples) {
 
     @Override
     public Object update(Object stored) {
-        Object ret = stored;
+        Object ret = (stored == null) ? this.agg.init() : stored;
         for(TridentTuple t: tuples) {
            ret =  this.agg.reduce(ret, t);
         }
         return ret;
     }        
-}
+}