Default to creating BOLT4 tlv payload format onions

Default to creating tlv onions for nodes for which we haven't received
any features through node announcements, and where no other features are
known such as invoice features nor channel init features.

Author: ViktorT-11

lightning/src/routing/router.rs

@@ -1101,7 +1101,9 @@ where L::Target: Logger {
 		} }
 	}
 
-	let empty_node_features = NodeFeatures::empty();
+	let mut variable_length_onion_optional_features = NodeFeatures::empty();
+	variable_length_onion_optional_features.set_variable_length_onion_optional();
+
 	// Find ways (channels with destination) to reach a given node and store them
 	// in the corresponding data structures (routing graph etc).
 	// $fee_to_target_msat represents how much it costs to reach to this node from the payee,
@@ -1132,7 +1134,7 @@ where L::Target: Logger {
 				let features = if let Some(node_info) = $node.announcement_info.as_ref() {
 					&node_info.features
 				} else {
-					&empty_node_features
+					&variable_length_onion_optional_features
 				};
 
 				if !features.requires_unknown_bits() {
@@ -1330,7 +1332,7 @@ where L::Target: Logger {
 							if let Some(node_info) = node.announcement_info.as_ref() {
 								ordered_hops.last_mut().unwrap().1 = node_info.features.clone();
 							} else {
-								ordered_hops.last_mut().unwrap().1 = NodeFeatures::empty();
+								ordered_hops.last_mut().unwrap().1 = variable_length_onion_optional_features.clone();
 							}
 						} else {
 							// We can fill in features for everything except hops which were