kb: protect against empty num_times

welco/kb/views.py

@@ -170,6 +170,8 @@ class KbZone(TemplateView):
         context['tags'] = Tag.objects.all().annotate(
                 num_times=Count('taggit_taggeditem_items')).filter(num_times__gt=0)
         num_times = context['tags'].values_list('num_times', flat=True)
+        if not num_times:
+            num_times = [0]
         delta = max(num_times) - min(num_times)
         for tag in context['tags']:
             if delta == 0: