[pipeline] allow image-text-to-text generate from text inputs

[zucchini-nlp]

What does this PR do?

As per title, passing empty list of images triggers image_processor call which in turn fails. It's because we usually check for Noneness in processors

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[yonigozlan]

Hello @zucchini-nlp ! I have this PR that I completely forgot about #35616, but it looks like it solves this issue. Added your tests to it, sorry about that!

[zucchini-nlp]

@yonigozlan oh oke, cool. Can it be merged soon? Gemma3 team was asking why it doesn't work so would be nice to include in next release

[yonigozlan]

Yes I asked for core maintainer approval, should be merged soon :)