Detect splice out as local contribution

Although a splice out doesn't include inputs to sign, users still need
to call back with `funding_transaction_signed` to sign the shared splice
input.

Author: wpaulino

lightning/src/ln/channelmanager.rs

@@ -9034,8 +9034,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			.and_then(|_| channel.interactive_tx_signing_session.as_mut())
 			.filter(|signing_session| signing_session.holder_tx_signatures().is_none())
 		{
-			let local_inputs_count = signing_session.local_inputs_count();
-			if local_inputs_count > 0 {
+			if signing_session.has_local_contribution() {
 				let mut pending_events = self.pending_events.lock().unwrap();
 				let unsigned_transaction = signing_session.unsigned_tx().build_unsigned_tx();
 				let event_action = (

lightning/src/ln/interactivetxs.rs

@@ -561,6 +561,24 @@ impl InteractiveTxSigningSession {
 			.count()
 	}
 
+	fn local_outputs_count(&self) -> usize {
+		self.unsigned_tx
+			.outputs
+			.iter()
+			.enumerate()
+			.filter(|(_, output)| {
+				!is_serial_id_valid_for_counterparty(
+					self.unsigned_tx.holder_is_initiator,
+					output.serial_id,
+				)
+			})
+			.count()
+	}
+
+	pub fn has_local_contribution(&self) -> bool {
+		self.local_inputs_count() > 0 || self.local_outputs_count() > 0
+	}
+
 	pub fn shared_input(&self) -> Option<&NegotiatedTxInput> {
 		self.unsigned_tx
 			.shared_input_index