check if source is nil, return UNKNOWN in that case

lib/RequestHandler.go

@@ -104,7 +104,7 @@ func ClickstreamJsonEventHandler(request RequestObject, topic string) {
 
 func getSource(event map[string]interface{}) string {
 	if event["source"] == nil {
-		event["source"] = "UNKNOWN"
+		return "UNKNOWN"
 	}
 	return event["source"].(string)
 }