[SDXL] Update ControlNet training example to zero out embeds for empty captions

[bghira]

What does this PR do?

Fixes #7920

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• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

@sayakpaul @yiyixuxu @linoytsaban

[sayakpaul]

This only becomes effective, though when you enable proportion_empty_prompt which I agree is important for ControlNet training.

[HuggingFaceDocBuilderDev]

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