refactor: Allow for client identities with no user ID

src/api/client/to_device.rs

@@ -22,7 +22,7 @@ pub(crate) async fn send_event_to_device_route(
 	State(services): State<crate::State>,
 	body: Ruma<send_event_to_device::v3::Request>,
 ) -> Result<send_event_to_device::v3::Response> {
-	let sender_user = body.identity.sender_user();
+	let sender_user = body.identity.expect_sender_user()?;
 	let sender_device = body.identity.sender_device();
 
 	// Check if this is a new transaction id