Fixed iterating by NoneType object

```
Traceback (most recent call last):
  File "/home/sentry/envs/sentry/lib/python2.7/site-packages/sentry/utils/safe.py", line 16, in safe_execute
    result = func(*args, **kwargs)
  File "/home/sentry/envs/sentry/lib/python2.7/site-packages/sentry/plugins/bases/notify.py", line 184, in post_process
    if not self.should_notify(group, event):
  File "/home/sentry/envs/sentry/lib/python2.7/site-packages/sentry/plugins/bases/notify.py", line 168, in should_notify
    if not any(v in allowed_tags.get(k) for k, v in tags):
  File "/home/sentry/envs/sentry/lib/python2.7/site-packages/sentry/plugins/bases/notify.py", line 168, in <genexpr>
    if not any(v in allowed_tags.get(k) for k, v in tags):
TypeError: argument of type 'NoneType' is not iterable
Error processing 'post_process' on 'MailProcessor': argument of type 'NoneType' is not iterable
```