fix retrieve call cdr by sid return null

lib/rest/rest-entity.js

@@ -34,7 +34,7 @@ class RestEntity {
     try {
       const res = await this.get(`Accounts/${this.accountSid}/${this.name}/${sid}`);
       //get call by sid will return call cdr as a object and not an array
-      return res ? res : null;
+      return res;
     } catch (err) {
       if (err.statusCode === 404) throw new Error(`${this.name}.retrieve: sid ${sid} not found`);
       if (err.statusCode !== 204) throw new Error(`${this.name}.retrieve: unexpected status code ${err.statusCode}`);