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d2adec38 - [Opensearch] : Fix the service validation in http_auth (#8609)

[Opensearch] : Fix the service validation in http_auth (#8609) ### Description OpenSearch supports validation using both Master Credentials (Username and password) and IAM. For Master Credentials users will not pass the argument `service` in `http_auth` and the existing code will break. To fix this, I have updated the condition to check if service attribute is present in http_auth before accessing it. ### Maintainers @baskaryan @navneet1v Signed-off-by: Naveen Tatikonda <navtat@amazon.com>