Potential error in `comfy/model_detection.py`

In function `convert_config`, there is a code snippet:

```python
if "attention_resolutions" in new_config:
attention_resolutions = new_config.pop("attention_resolutions")
transformer_depth = new_config.get("transformer_depth", None)
transformer_depth_middle = new_config.get("transformer_depth_middle", None)

if isinstance(transformer_depth, int):
transformer_depth = len(channel_mult) * [transformer_depth]
if transformer_depth_middle is None:
transformer_depth_middle = transformer_depth[-1]
```

If both `transformer_depth` and `transformer_depth_middle` are set to `None`, `transformer_depth[-1]` will be accessed, which is invalid as `transformer_depth` is `None`.