extra condition to avoid print info of NoneType object (#35)

Author: José Carlos Ferreiro

pypingdom/maintenance.py

@@ -36,7 +36,7 @@ def __repr__(self):
                    ", ".join(checks))
 
     def to_json(self):
-        check_ids = [str(check._id) for check in self.checks]
+        check_ids = [str(check._id) for check in self.checks if check]
         data = {
             # "__csrf_magic": "",
             # "id": "",