Hotfix for failing `MusicgenForConditionalGeneration` tests
# What does this PR do?

A exceptional case (for `MusicgenForConditionalGeneration`) is not detected by the CI triggered in #24927.

Test is currently failing on `main`, see

https://app.circleci.com/pipelines/github/huggingface/transformers/68971/workflows/503866ea-e425-4817-9849-52c6e7fd2865/jobs/863722
_The documentation is not available anymore as the PR was closed or merged._