Make Trampoline inbound MPP data optional

Per the resolution of the spec discussion regarding the requirement of
this field outside MPP scenarios, we are now marking this field as
optional.

Author: arik-so

lightning/src/ln/msgs.rs

@@ -2078,7 +2078,7 @@ mod fuzzy_internal_msgs {
 	pub struct InboundTrampolineEntrypointPayload {
 		pub amt_to_forward: u64,
 		pub outgoing_cltv_value: u32,
-		pub multipath_trampoline_data: FinalOnionHopData,
+		pub multipath_trampoline_data: Option<FinalOnionHopData>,
 		pub trampoline_packet: TrampolineOnionPacket,
 		/// The blinding point this hop needs to decrypt its Trampoline onion.
 		/// This is used for Trampoline hops that are not the blinded path intro hop.
@@ -3272,7 +3272,7 @@ impl<NS: Deref> ReadableArgs<(Option<PublicKey>, NS)> for InboundOnionPayload wh
 			return Ok(Self::TrampolineEntrypoint(InboundTrampolineEntrypointPayload {
 				amt_to_forward: amt.ok_or(DecodeError::InvalidValue)?,
 				outgoing_cltv_value: cltv_value.ok_or(DecodeError::InvalidValue)?,
-				multipath_trampoline_data: payment_data.ok_or(DecodeError::InvalidValue)?,
+				multipath_trampoline_data: payment_data,
 				trampoline_packet: trampoline_onion_packet,
 				current_path_key: intro_node_blinding_point,
 			}))