fix issue with mailbox count calculation

- use 0 if no mailbox option is assigned to the hosting package

gnuviechadmin/hostingpackages/models.py

@@ -255,7 +255,7 @@ class CustomerHostingPackage(HostingPackageBase):
         ).aggregate(
             mailbox_sum=models.Sum('number')
         )
-        return self.mailboxcount + result['mailbox_sum']
+        return self.mailboxcount + (result['mailbox_sum'] or 0)
 
     def get_databases(self):
         """