invoices: exit early when the subscriber chan is nil

When calling `NotifyExitHopHtlc` it is allowed to pass a chan to
subscribe to the HTLC's resolution when it's settled. However, this
method will also return immediately if there's already a resolution,
which means it behaves like a notifier and a getter. If the caller
decides to only use the getter to do a non-blocking lookup, it can pass
a nil subscriber chan to bypass the notification.

contractcourt/mock_registry_test.go

@@ -29,6 +29,11 @@ func (r *mockRegistry) NotifyExitHopHtlc(payHash lntypes.Hash,
 	wireCustomRecords lnwire.CustomRecords,
 	payload invoices.Payload) (invoices.HtlcResolution, error) {
 
+	// Exit early if the notification channel is nil.
+	if hodlChan == nil {
+		return r.notifyResolution, r.notifyErr
+	}
+
 	r.notifyChan <- notifyExitHopData{
 		hodlChan:      hodlChan,
 		payHash:       payHash,

invoices/invoiceregistry.go

@@ -1275,7 +1275,11 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 			invoiceToExpire = makeInvoiceExpiry(ctx.hash, invoice)
 		}
 
-		i.hodlSubscribe(hodlChan, ctx.circuitKey)
+		// Subscribe to the resolution if the caller specified a
+		// notification channel.
+		if hodlChan != nil {
+			i.hodlSubscribe(hodlChan, ctx.circuitKey)
+		}
 
 	default:
 		panic("unknown action")