Minor: concisely construct RecipientOnionFields

We'll basically be duplicating this handling code in an upcoming patch, so want
to make it concise.

Author: valentinewallace

lightning/src/ln/outbound_payment.rs

@@ -1169,11 +1169,7 @@ impl OutboundPayments {
 						},
 						PendingOutboundPayment::InvoiceReceived { payment_hash, retry_strategy, .. } => {
 							let total_amount = route_params.final_value_msat;
-							let recipient_onion = RecipientOnionFields {
-								payment_secret: None,
-								payment_metadata: None,
-								custom_tlvs: vec![],
-							};
+							let recipient_onion = RecipientOnionFields::spontaneous_empty();
 							let retry_strategy = Some(*retry_strategy);
 							let payment_params = Some(route_params.payment_params.clone());
 							let (retryable_payment, onion_session_privs) = self.create_pending_payment(