[ML][Pipelines] Fix parallel-for validation rule: allow empty dict as loop config item

[D-W-]

Description

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In parallel-for scenarios, when loop body don't take any arguments, the loop config's value will be None.
For example, the following example expects to execute loop body twice.
Since basic_component doesn't take any arguments, it's config is an empty dict

        @pipeline
        def valid_pipeline():
            body = basic_component()
            parallel_for(
                body=body,
                items=[{}, {}],
            )

        valid_pipeline()

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[azure-sdk]

API change check

API changes are not detected in this pull request.