Mark the callback as optional

Without callback, it is impossible to unit-test the backbeat
routes, as we run async code and do not yet support promises.

Issue: CLDSRV-591

lib/routes/routeBackbeat.js

@@ -1310,7 +1310,8 @@ const indexingSchema = joi.array().items(indexEntrySchema).min(1);
 
 function respondToRequest(err, response, log, callback) {
     responseJSONBody(err, null, response, log);
-    return callback(err);
+    // The callback is optional, as it is only used for testing purposes
+    return callback?.(err);
 }
 
 function routeIndexingAPIs(request, response, userInfo, log, callback) {