provider: Allow empty string in assume_role.role_arn

[gdavison]

Description

For backwards compatibility, defining an assume_role block with a null or empty role_arn is supported, and does not attempt to assume a role. The PR #39255 did not check the empty string case.

Relations

Closes #39296.
Closes #39306.
Closes #39331.

[github-actions]

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[ewbankkit]

LGTM 🚀.

% go test ./internal/provider 
ok  	github.com/hashicorp/terraform-provider-aws/internal/provider	7.804s

[github-actions]

This functionality has been released in v5.68.0 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!