fix(filter_policy_scope): changed default to null, added tests

[vladget]

what

• Bug fix for default value of filter_policy_scope
• Added test to avoid a similar bug in the future

why

the default value(MessageAttributes) of filter_policy_scope cause error in the case when filter_policy is unset :

│ Error: filter_policy is required when filter_policy_scope is set
│ 
│   with module.sns_topic["my-topic"].aws_sns_topic_subscription.this["my-topic"],
│   on .terraform/modules/sns_topic/main.tf line 29, in resource "aws_sns_topic_subscription" "this":
│   29: resource "aws_sns_topic_subscription" "this" {

[osterman]

/terratest

[goruha]

/terratest

[github-actions]

These changes were released in v1.1.0.