fix: add tags if the default route table association is enabled

[michelzanini]

Description

Fixes #51

Motivation and Context

Attempts to fix this error:

Error: error creating EC2 Tag (ops.company.cloud/provisioner) for resource (): error tagging resource (): InvalidParameterValue: Value ( null ) for parameter resourceId is invalid. Null/empty value for resourceId is invalid
	status code: 400, request id: 42192f9f-1546-4714-b9b2-b5b4e948ef64

  on .terraform/modules/tgw/main.tf line 44, in resource "aws_ec2_tag" "this":
  44: resource "aws_ec2_tag" "this" {

This would happen when the transit gateway has enable_default_route_table_association as false.

How Has This Been Tested?

[michelzanini]

Hi @tfhartmann ,
This PR attempts to fix #51 .
Maybe you would be able to test this works for you?
Thanks.

[bpalermo]

@antonbabenko can we get a review here? I'm also hitting this issue.

Tks!

[tfhartmann]

This looks good to me!

[tfhartmann]

🎉

[github-actions]

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