Sygnal assumes that the sender field of a notification is non-null

*This issue was originally created by [**@anoadragon453**](https://github.com/anoadragon453) at .*

https://github.com/matrix-org/sygnal/blob/ab6285760d76d345edb78673106d5913cfe69259/sygnal/apnspushkin.py#L343-L346

According to [the spec](https://matrix.org/docs/spec/push_gateway/r0.1.1#post-matrix-push-v1-notify) the `sender` field of a notification can be null, yet Sygnal looks to assume it's a str.

While not common, if sent by a homeserver this would cause Sygnal to return a 500.