Add missing optional option on the check that captures the reward

This shouldn't be an error, it's an expected situation as described in the article.

testing-modules/load-testing-comparison/src/main/resources/scripts/Gatling/GatlingScenario.scala

@@ -28,7 +28,7 @@ class RewardsScenario extends Simulation {
 		
 		.exec(http("get_reward")
 		  .get("/rewards/find/${custId}")
-		  .check(jsonPath("$.id").saveAs("rwdId")))
+		  .check(jsonPath("$.id").optional.saveAs("rwdId")))
 		.pause(1)
 		
 		.doIf("${rwdId.isUndefined()}"){