fix: honor empty processor_kwargs={} in multimodal pipelines (#48044)

* fix: treat empty processor_kwargs as explicit

`processor_kwargs={}` was falsy under `or processing_kwargs`, so an
explicit empty dict incorrectly fell back to the full processing kwargs
and could leak unrelated arguments into the processor call.

* test: cover empty processor_kwargs in image-text-to-text preprocess

* test the actual warning, no monkey patch

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Co-authored-by: raushan <raushan@huggingface.co>