r/aws_ssm_service_setting: fix panic on empty result

[kamilturek]

Description

This PR fixes a panic happening on an empty service setting status result.

Relations

Closes #27225.

Output from Acceptance Testing

$ make testacc TESTARGS='-run=TestAccSSMServiceSetting_' PKG=ssm 
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ssm/... -v -count 1 -parallel 20  -run=TestAccSSMServiceSetting_ -timeout 180m
=== RUN   TestAccSSMServiceSetting_basic
=== PAUSE TestAccSSMServiceSetting_basic
=== CONT  TestAccSSMServiceSetting_basic
--- PASS: TestAccSSMServiceSetting_basic (51.23s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/ssm        52.976s

[github-actions]

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[ewbankkit]

LGTM 🚀.

% make testacc TESTARGS='-run=TestAccSSMServiceSetting_' PKG=ssm ACCTEST_PARALLELISM=3
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ssm/... -v -count 1 -parallel 3  -run=TestAccSSMServiceSetting_ -timeout 180m
=== RUN   TestAccSSMServiceSetting_basic
=== PAUSE TestAccSSMServiceSetting_basic
=== CONT  TestAccSSMServiceSetting_basic
--- PASS: TestAccSSMServiceSetting_basic (33.37s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ssm	37.365s

[ewbankkit]

@kamilturek Thanks for the contribution 🎉 👏.

[github-actions]

This functionality has been released in v4.35.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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