Setting notify_arns does not set {alarm,ok,insufficient_data}_actions

[LukasdeBoer]

Describe the Bug

This module exposes 4 different variables to set ARNs that the cloudwatch alarms will set as their actions:

• notify_arns
• alarm_actions
• ok_actions
• insufficient_data_actions

The idea being that when you only set notify_arns, it will set the other action arns for you, so you don't have to pass the same variable three times. This works using a coalescelist on one of the actions, and the notify_arn, to get the arn it needs (https://github.com/cloudposse/terraform-aws-alb-target-group-cloudwatch-sns-alarms/blob/master/main.tf#L43)

# default to using notify_arns unless a more specific action is specified.
alarm_actions             = compact(coalescelist(var.alarm_actions, var.notify_arns))
ok_actions                = compact(coalescelist(var.ok_actions, var.notify_arns))
insufficient_data_actions = compact(coalescelist(var.insufficient_data_actions, var.notify_arns))

This worked fine, until #18 was merged. This PR changed the default value of var.alarm_actions, var.ok_actions and var.insufficient_data_actions from [] to [""], giving coalescelist a significantly different behaviour.

According to the terraform docs, coalescelist takes any number of list arguments and returns the first one that isn't empty. However, it does not consider a list containing an empty string as an empty list:

terraform console:

coalescelist([], [""], ["test"])
[
  "",
]

We would like to get "test" returned here, but in fact it returns the array containing an empty string.

Expected Behavior

I expect that when you set an ARN as notify_arns, and do not set alarm_actions, ok_actions and insufficient_data_actions, that the module still sets the alarms alarm_actions, ok_actions, and insufficient_data_actions to notify_arns.

Steps to Reproduce

Set an ARN as notify_arns, do not set alarm_actions, ok_actions and insufficient_data_actions. This module will create cloudwatch alarms without actions. If you pass the arn as alarm_actions, ok_actions and insufficient_data_actions, it will set the alarms.

Environment (please complete the following information):

Tried this module's latest version (0.15.0) with latest Terraform 0.13.7, 0.14.11, 0.15.5.