fixed bug that if sample size was zero a type error would be thrown. …

…In practice no datasets should ever be published and have no subjects but still a bad logic failure.

open_fmri/apps/dataset/views.py

@@ -298,6 +298,7 @@ def get_context_data(self, **kwargs):
         context['num_datasets'] = len(Dataset.objects.filter(status='PUBLISHED'))
         total = 0
         for x in Dataset.objects.filter(status='PUBLISHED'):
-            total += x.sample_size
+            if x.sample_size is not None:
+                total += x.sample_size
         context['num_subjects'] =  total
         return context