[Fix] Explicitly check if output['generation_logits'] is an empty list

[fjosw]

When running a GenerationSession with gather_generation_logits and the first token that is generated has end_id then outputs['generation_logits'] is an empty list. This crashes the session in _prepare_outputs https://github.com/NVIDIA/TensorRT-LLM/blob/3d56a445e8ebf888e78be638faf6beec0a78f3c2/tensorrt_llm/runtime/model_runner.py#L253 with

RuntimeError: stack expects a non-empty TensorList

This PR adds an explicit check and leaves the empty list unaltered. Alternatively one could also delete the dict entry or return an empty tensor in this edge case.