corrects single file path validation logic

[andrew-w-ross]

What does this PR do?

Fix's loading ModularPipelines loading component specs where pretrained_model_name_or_path is a path.

Didn't bother opening an issue considering the tiny size of the fix.

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[DN6]

Thank you @andrew-w-ross! Small change requested to the test, but good to merge once that's updated 👍🏽

[andrew-w-ross]

@DN6 Thanks for the review, I accepted your test changes. Is there anything else I need to do on my side to get this merged? I'm just seeing 5 workflows awaiting approval.

[HuggingFaceDocBuilderDev]

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