[s3-event-default-constructor] Adding default constructor for S3Event for easier deserialization

Author: gal bashan

aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/S3Event.java

@@ -15,6 +15,12 @@ public class S3Event extends S3EventNotification implements Serializable, Clonea
 
     private static final long serialVersionUID = -8094860465750962044L;
 
+    /**
+     * default constructor
+     * (Not available in v1)
+     */
+    public S3Event() {}
+
     /**
      * Create a new instance of S3Event
      * @param records A list of S3 event notification records