dont fail req deserialisation over empty push response body

Signed-off-by: strawberry <strawberry@puppygock.gay>

crates/ruma-push-gateway-api/src/send_event_notification.rs

@@ -48,6 +48,7 @@ pub mod v1 {
         /// pushkeys and remove the associated pushers. It may not necessarily be the notification
         /// in the request that failed: it could be that a previous notification to the same
         /// pushkey failed. May be empty.
+        #[serde(default)]
         pub rejected: Vec<String>,
     }