fix resource aggregations when not found dataset

orchestra/contrib/resources/aggregations.py

@@ -34,6 +34,7 @@ class Last(Aggregation):
     def filter(self, dataset, date=None):
 
         lastdataset = dataset.order_by('-id').first()
+        if lastdataset is not  None:
             dataset = dataset.filter( launch_id=lastdataset.launch_id)
         # now = timezone.now()
         # epoch = now - datetime.timedelta(minutes=2)