Fixes a key error where the mailer tried to get the ``msgtype`` of an
event that was missing a ``msgtype``.
```
File "synapse/push/mailer.py", line 264, in get_notif_vars
File "synapse/push/mailer.py", line 285, in get_message_vars
File ".../frozendict/__init__.py", line 10, in __getitem__
return self.__dict[key]
KeyError: 'msgtype'
```