contractcourt+lnd: make IncubateOutputs take fn.Option

`IncubateOutputs` never takes more than one HTLC, so we change the
params to be optional, which helps with the following commit where we
pass the deadline height when incubating outgoing HTLCs.

server.go

@@ -38,6 +38,7 @@ import (
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/feature"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -1148,23 +1149,12 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			return nil
 		},
 		IncubateOutputs: func(chanPoint wire.OutPoint,
-			outHtlcRes *lnwallet.OutgoingHtlcResolution,
-			inHtlcRes *lnwallet.IncomingHtlcResolution,
+			outHtlcRes fn.Option[lnwallet.OutgoingHtlcResolution],
+			inHtlcRes fn.Option[lnwallet.IncomingHtlcResolution],
 			broadcastHeight uint32) error {
 
-			var (
-				inRes  []lnwallet.IncomingHtlcResolution
-				outRes []lnwallet.OutgoingHtlcResolution
-			)
-			if inHtlcRes != nil {
-				inRes = append(inRes, *inHtlcRes)
-			}
-			if outHtlcRes != nil {
-				outRes = append(outRes, *outHtlcRes)
-			}
-
 			return s.utxoNursery.IncubateOutputs(
-				chanPoint, outRes, inRes,
+				chanPoint, outHtlcRes, inHtlcRes,
 				broadcastHeight,
 			)
 		},