Convert None logits processor/stopping criteria to empty list.
Author: ccmaymay
Pull request: yes

# What does this PR do?

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Fixes #19876 (TypeError from GenerationMixin.generate() when stopping_criteria is None)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Thank you for the feedback @gante!  I've made the requested changes.  I was going to add tests, but then I realized that by changing the defaults to `None`, many existing tests implicitly check that `None` is allowable. 😊