Fix TypeError: 'NoneType' object is not iterable in GenerationMixin.generate

[albertvillanova]

What does this PR do?

Fix TypeError: 'NoneType' object is not iterable in GenerationMixin.generate

• Fix for None layer_types

Fixes issue introduced in:

• 🚨 [Cache] Native mamba & hybrid cache #44950

See the corresponding downstream issue in trl:

• CI fails with dev dependencies: TypeError: 'NoneType' object is not iterable trl#5425

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View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=45164&sha=1e5271