`apply_chat_template` crashes with `IndexError` when passed an empty list

### System Info transformers: 5.13.0.dev0 (latest main, commit 2b422bde64) Python: 3.14.4 OS: Ubuntu 25.04 (WSL2) ### Who can help? _No response_ ### Information - [ ] The official example scripts - [ ] My own modified scripts ### Tasks - [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...) - [ ] My own task or dataset (give details below) ### Reproduction ```python from transformers import AutoTokenizer tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-1.5B-Instruct") tokenizer.apply_chat_template([], tokenize=False) ``` Traceback: ``` File "tokenization_utils_base.py", line 3090, in apply_chat_template isinstance(conversation[0], (list, tuple)) or hasattr(conversation[0], "messages") ~~~~~~~~~~~~^^^ IndexError: list index out of range ``` Also reproduced on `google/gemma-2-2b-it` — same `IndexError`. ### Expected behavior Should raise a clear `ValueError` (e.g., "conversation must not be empty") instead of an unhandled `IndexError`. Root cause: In `tokenization_utils_base.py` line 3090, `conversation[0]` is accessed without checking if the list is empty: ```python if isinstance(conversation, (list, tuple)) and ( isinstance(conversation[0], (list, tuple)) or hasattr(conversation[0], "messages") ): ``` Fix: add `len(conversation) > 0` guard before accessing index 0.