GCMPushkin assumes that content field is not null. Fails when push format is event_id_only

[octopop]

If the pusher is set with format:'event_id_only' per the spec, no content is necessary and the pusher may omit it for any reason:

The content field from the event, if present. The pusher may omit this if the event had no content or for any other reason.
https://spec.matrix.org/v1.9/push-gateway-api/#post_matrixpushv1notify

With above; Sygnal fails with error below. It needs to check if data["contents"] is not None before attempting to iterate over it.

│ 2024-03-09 06:36:18,785 [1] DEBUG sygnal.http [f54170f3-1882-4626-b3fd-accc51c10842] Sending push to pushkin com.example.myappsynapsenotification for app ID com.example.myappiosynapsenotificatio │
│ 2024-03-09 06:36:18,786 [1] ERROR sygnal.http [f54170f3-1882-4626-b3fd-accc51c10842] Exception whilst dispatching notification.                                                                        │
│ Traceback (most recent call last):                                                                                                                                                                     │
│   File "/usr/local/lib/python3.10/site-packages/sygnal/http.py", line 275, in _handle_dispatch                                                                                                         │
│     result = await pushkin.dispatch_notification(notif, d, context)                                                                                                                                    │
│   File "/usr/local/lib/python3.10/site-packages/sygnal/notifications.py", line 217, in dispatch_notification                                                                                           │
│     return await self._dispatch_notification_unlimited(n, device, context)                                                                                                                             │
│   File "/usr/local/lib/python3.10/site-packages/sygnal/gcmpushkin.py", line 523, in _dispatch_notification_unlimited                                                                                   │
│     data = GcmPushkin._build_data(n, device, self.api_version)                                                                                                                                         │
│   File "/usr/local/lib/python3.10/site-packages/sygnal/gcmpushkin.py", line 676, in _build_data                                                                                                        │
│     for attr, value in data["content"].items():                                                                                                                                                        │
│ AttributeError: 'NoneType' object has no attribute 'items'