What does this PR do?

Avoid this when Grounding Dino returns no result:

list(zip(results["boxes"], results["labels"], results["scores"], strict=True))
# ValueError: zip() argument 2 is longer than argument 1

I tried to fix it at tokenizer level first but other models (e.g. Mistral) expect the empty string.

Code Agent Policy

• I confirm that this is not a pure code agent PR.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

maybe @yonigozlan or @zucchini-nlp