r/aws_iot_thing_group: fix crash with empty attribute_payload block

[jar-b]

Description

Setting the attribute_payload argument to an empty block resulted the in the associated expander attempting to assert the lists single nil item into a map. This change adds a check for nil items to prevent this.

Relations

Closes #38093

Output from Acceptance Testing

% make testacc PKG=iot TESTS=TestAccIoTThingGroup_
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.22.6 test ./internal/service/iot/... -v -count 1 -parallel 20 -run='TestAccIoTThingGroup_'  -timeout 360m

--- PASS: TestAccIoTThingGroup_disappears (12.93s)
--- PASS: TestAccIoTThingGroup_basic (15.24s)
--- PASS: TestAccIoTThingGroup_parentGroup (19.73s)
--- PASS: TestAccIoTThingGroup_properties (22.78s)
--- PASS: TestAccIoTThingGroup_tags (30.27s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/iot        36.178s

[github-actions]

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[ewbankkit]

LGTM 🚀.

[github-actions]

This functionality has been released in v5.63.0 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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