resource/aws_sns_topic_subscription: Always set Terraform state on reads, fix filter_policy removal

[bflad]

Fixes #1910
Fixes #3328

Changes proposed in this pull request:

• Always call d.Set() for all attributes on read
• Send "{}" on filter_policy removal as noted by https://docs.aws.amazon.com/sns/latest/dg/message-filtering.html#message-filtering-policy-remove

Previously (before code updates):

--- FAIL: TestAccAWSSNSTopicSubscription_basic (10.89s)
    testing.go:527: Step 0 error: Check failed: Check 3/8 error: aws_sns_topic_subscription.test_subscription: Attribute 'delivery_policy' not found
--- FAIL: TestAccAWSSNSTopicSubscription_filterPolicy (28.36s)
    testing.go:527: Step 3 error: Error applying: 1 error occurred:
        	* aws_sns_topic_subscription.test_subscription: 1 error occurred:
        	* aws_sns_topic_subscription.test_subscription: error setting subscription (arn:aws:sns:us-west-2:187416307283:terraform-test-topic-7447862065639079919:d0385467-b42a-4018-a22f-cdd19066fe18) attribute (FilterPolicy): InvalidParameter: Invalid parameter: FilterPolicy: Filter is not an object

Output from acceptance testing:

--- PASS: TestAccAWSSNSTopicSubscription_basic (15.92s)
--- PASS: TestAccAWSSNSTopicSubscription_filterPolicy (34.20s)
--- PASS: TestAccAWSSNSTopicSubscription_deliveryPolicy (33.09s)
--- PASS: TestAccAWSSNSTopicSubscription_rawMessageDelivery (31.41s)
--- PASS: TestAccAWSSNSTopicSubscription_autoConfirmingEndpoint (44.73s)
--- PASS: TestAccAWSSNSTopicSubscription_autoConfirmingSecuredEndpoint (75.19s)

[ghost]

This has been released in version 1.39.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "aws" {
	version = "~> 1.39.0"
}
# ... other configuration ...

[ghost]

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