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.

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")