return quickly if there is nothing to aggregate

autovpn/acct.py

@@ -51,6 +51,10 @@ class Accounting(object):
                 aggr_day[attr] += conn_info[attr]
         pipe.execute()
 
+        # Short return if there's nothing to do.
+        if not aggr:
+            return
+
         # Aggregate values on the master server.
         days = aggr.keys()
         aggr_key = 'aggr:%s' % cn