fix: Use an empty map as default value for default_capacity_provider_strategy

[pwillis-oi]

If the default value is null, the module fails with Error: Iteration over null value. This changes the default from null to an empty map to avoid this error.

Description

The default null value for default_capacity_provider_strategy always causes the module to fail.

│ Error: Iteration over null value
│
│   on /var/folders/7s/4m_zx_dj4616qf_rrf5wr1km0000gn/T/tfsh.aa9dd864fd/modules/ecs_cluster/modules/cluster/main.tf line 105, in resource "aws_ecs_cluster_capacity_providers" "this":
│  105:     [for k, v in var.default_capacity_provider_strategy : try(coalesce(v.name, k))],
│     ├────────────────
│     │ var.default_capacity_provider_strategy is null
│
│ A null value cannot be used as the collection in a 'for' expression.

Changing this to an empty map allows the module to continue without errors.

Motivation and Context

• Resolves default_capacity_provider_strategy can NOT be null (which is the default value) #324

Breaking Changes

Should not be a breaking change since the module was breaking with the default as-is, but I have not done any regression testing...

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

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

I have tested this on a module I already have in use that I'm upgrading to the 6.x module version. I never specified that variable before, so the default is being used, and the module breaks. This fix avoids the error

[pwillis-oi]

Also, this is out of scope of the PR, but if the authors want, I can open a separate PR with a POC of running regression tests on PRs. This GitHub Action is what I use to run a matrix of jobs, each using a different Terraform release, to run a suite of simple tests. In this way I can see if any of my tests fail on a different version of Terraform. The same could be done with provider versions for example.

[bryantbiggs]

Also, this is out of scope of the PR, but if the authors want, I can open a separate PR with a POC of running regression tests on PRs. This GitHub Action is what I use to run a matrix of jobs, each using a different Terraform release, to run a suite of simple tests. In this way I can see if any of my tests fail on a different version of Terraform. The same could be done with provider versions for example.

we already do this in our CI - testing lower and upper bounds of Terraform versions

[antonbabenko]

This PR is included in version 6.1.1 🎉

[github-actions]

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