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Describe the bug

The SGLang router fails to start if it is launched without the --worker-urls command-line argument. This prevents the router from being initialized without a predefined set of workers, which is a valid use case (e.g., when workers will be registered dynamically).

The error was introduced on this commit: 09ae5b2

File: sgl-router/py_src/sglang_router/launch_router.py:208

Proposed solution:
The issue can be resolved by modifying RouterArgs.add_cli_args in sglang/sgl-router/py_src/sglang_router/launch_router.py to ensure --worker-urls defaults to an empty list.

# In RouterArgs.add_cli_args:
parser.add_argument(
    "--worker-urls",
    type=str,
    nargs="*",  # Allow zero or more arguments
    default=[], # Default to an empty list
    help="List of worker URLs (e.g., http://worker1:8000 http://worker2:8000)",
)```

### Reproduction

1. Run the router launcher from the command line without providing any worker URLs.

`python -m sglang_router.launch_router --port 30000 --host 0.0.0.0 --policy random`

The router crashes with a TypeError because the underlying Rust component receives None instead of a list for the worker_urls parameter.

Error starting router: argument 'worker_urls': 'NoneType' object cannot be converted to 'Sequence'
Traceback (most recent call last):
File ".../sglang_router/launch_router.py", line 473, in main
launch_router(router_args)
File ".../sglang_router/launch_router.py", line 392, in launch_router
router = Router(...)
File ".../sglang_router/router.py", line 90, in init
self._router = _Router(...)
TypeError: argument 'worker_urls': 'NoneType' object cannot be converted to 'Sequence'

### Environment

This is an error on the launch script on version 0.1.5.