fix: Change subnets default value to null

[andyli]

Description

Change default of subnets from [] to null.

Motivation and Context

Fix #340.

Breaking Changes

No. As far as I know, the previous default of subnets = [] is useless, because

• the default will be overridden when subnets is provided by user, or
• the default will be ignored when subnet_mapping is provided by user.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
  • No change to examples were needed.
• I have tested and validated these changes using one or more of the provided examples/* projects
  • Tested with the code in "subnet_mapping": only one of subnet_mapping,subnets can be specified, but subnet_mapping,subnets were specified. #340.

• I have executed pre-commit run -a on my pull request

[bryantbiggs]

I don't believe this is accurate - ref: hashicorp/terraform-provider-aws#17281 (comment)

[andyli]

As I've mentioned in #340, the issue happens since AWS provider v5.31.0.

The current version of terraform-aws-alb stated the AWS provider version constraint as >= 5.31:

terraform-aws-alb/versions.tf

Lines 4 to 9 in 9ff4ea8

	required_providers {
	aws = {
	source = "hashicorp/aws"
	version = ">= 5.31"
	}
	}

The comment in #17281 was describing AWS provider v5.25.0. The "fix" of that landed in v5.31.0, which caused the issued I described in #340.

[antonbabenko]

This PR is included in version 9.4.1 🎉

[github-actions]

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