Use null as default value for target group ARNs (#524)

* Use null as default value for target group ARNs
* Update CHANGELOG

close #499

Author: tatusl

CHANGELOG.md

@@ -15,7 +15,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
-- Write your awesome change here (by @you)
+- Use null as default value for `target_group_arns` attribute of worker autoscaling group (by @tatusl)
 
 # History
 

local.tf

@@ -50,7 +50,7 @@ locals {
     iam_instance_profile_name     = ""                          # A custom IAM instance profile name. Used when manage_worker_iam_resources is set to false. Incompatible with iam_role_id.
     iam_role_id                   = "local.default_iam_role_id" # A custom IAM role id. Incompatible with iam_instance_profile_name.  Literal local.default_iam_role_id will never be used but if iam_role_id is not set, the local.default_iam_role_id interpolation will be used.
     suspended_processes           = ["AZRebalance"]             # A list of processes to suspend. i.e. ["AZRebalance", "HealthCheck", "ReplaceUnhealthy"]
-    target_group_arns             = []                          # A list of ALB target group ARNs to be associated to the ASG
+    target_group_arns             = null                        # A list of ALB target group ARNs to be associated to the ASG
     enabled_metrics               = []                          # A list of metrics to be collected i.e. ["GroupMinSize", "GroupMaxSize", "GroupDesiredCapacity"]
     placement_group               = ""                          # The name of the placement group into which to launch the instances, if any.
     service_linked_role_arn       = ""                          # Arn of custom service linked role that Auto Scaling group will use. Useful when you have encrypted EBS