Backport of actions: don't panic when resource change is nil - it might be a refresh into v1.4

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Backport

This PR is auto-generated from #37687 to be assessed for backporting due to the inclusion of the label 1.4-backport.

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@mildwonkey
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merge conflict error: POST https://api.github.com/repos/hashicorp/terraform/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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@steve-hashi reported a bug where any configuration with actions would panic when running terraform plan --refresh-only. This removes the panic, returning nil if no changes are found for the resource.

Fixes #

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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Overview of commits

• 24b5e7b

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