[BUG] Cannot run math_500/mmlu_pro on models without chat template

## Describe the bug
When running math_500/mmlu_pro (and potentially other tasks) on a model without a chat template, I get the following error:

```bash
TypeError: can only concatenate list (not "tuple") to list
```

[at the following line](https://github.com/huggingface/lighteval/blob/58038182c0f8090b4cb023b3400487b56ffdc948/src/lighteval/models/transformers/transformers_model.py#L692).

## To Reproduce

```bash
lighteval accelerate "model_name=HuggingFaceTB/SmolLM2-135M" "mmlu_pro|0"
```

## Expected behavior

Eval to run

## Version info

lighteval==0.13
transformers==4.57.3