aws_lambda: Code.from_asset requires string as the first argument.

### Describe the feature

Make Code.from_asset first argument optional.

### Use Case

The reason for this is when create a lambda dependency layer you have no reason to copy in files. To avoid copying in files I had to create an empty directory.

```python
code=Code.from_asset('empty',
bundling=BundlingOptions(
image=Runtime.PYTHON_3_11.bundling_image,
# Must follow this file structure
# https://docs.aws.amazon.com/lambda/latest/dg/python-layers.html
command=["bash", "-c", "pip install pygeohash==1.2.0 -t /asset-output/python/"]
```

### Proposed Solution

Either create a more specific Code.from_bundle or make the path argument optional.

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.142.1 (build ed4e152)

### Environment details (OS name and version, etc.)

Ubuntu 22.04, Python3.11

Start at the Code.from_asset entry point and trace how its first argument is validated when BundlingOptions is supplied. Compare the requested optional-path behavior with the existing API and tests; done means a dependency layer can be bundled from the shown command without creating an empty directory, while preserving current asset behavior.

Written by the indexing model from the issue text.