lnwallet: implement special case for OP_RETURN in rbf-coop

In this commit, we implement a special case for OP_RETURN scripts
outlined in the spec. If a party decides that its output will be too
small even after the dust check, then they can opt to set it to zero by sending an `OP_RETURN` as their script.

lnwallet/channel.go

@@ -9269,13 +9269,19 @@ func CreateCooperativeCloseTx(fundingTxIn wire.TxIn,
 		closeTx.LockTime = lockTime
 	})
 
-	// TODO(roasbeef): needs support for dropping inputs
-
-	// Create both cooperative closure outputs, properly respecting the
-	// dust limits of both parties.
+	// Create both cooperative closure outputs, properly respecting the dust
+	// limits of both parties.
 	var localOutputIdx fn.Option[int]
 	haveLocalOutput := ourBalance >= localDust
 	if haveLocalOutput {
+		// If our script is an OP_RETURN, then we set our balance to
+		// zero.
+		if opts.customSequence.IsSome() &&
+			input.ScriptIsOpReturn(ourDeliveryScript) {
+
+			ourBalance = 0
+		}
+
 		closeTx.AddTxOut(&wire.TxOut{
 			PkScript: ourDeliveryScript,
 			Value:    int64(ourBalance),
@@ -9287,6 +9293,14 @@ func CreateCooperativeCloseTx(fundingTxIn wire.TxIn,
 	var remoteOutputIdx fn.Option[int]
 	haveRemoteOutput := theirBalance >= remoteDust
 	if haveRemoteOutput {
+		// If a party's script is an OP_RETURN, then we set their
+		// balance to zero.
+		if opts.customSequence.IsSome() &&
+			input.ScriptIsOpReturn(theirDeliveryScript) {
+
+			theirBalance = 0
+		}
+
 		closeTx.AddTxOut(&wire.TxOut{
 			PkScript: theirDeliveryScript,
 			Value:    int64(theirBalance),