for study: visualize query result if there's no suffix

Author: zihan21206

runestone/hparsons/js/SQLFeedback.js

@@ -174,7 +174,7 @@ export default class SQLFeedback extends HParsonsFeedback {
                 // and visualize the error
                 this.visualizeResults(respDiv, this.results);
                 executionSuccessFlag = false;
-            } else if (query.suffix != null) {
+            } else if (!query.suffix) {
                 this.visualizeResults(respDiv, this.results);
             }
         }