Fix this.ping()

src/usimp.ts

@@ -328,7 +328,7 @@ export class Session {
                         'data': data
                     }));
 
-                    if (cb === null) {
+                    if (cb === undefined) {
                         const responseData = await response;
                         const endTime = performance.now();
                         responseData.duration = endTime - startTime;