[core] fix: normalize the cmdline field in StatsPayload schema

[rueian]

Description

Fix this:

2026-06-19 23:17:11,662	ERROR reporter_agent.py:1983 -- Error publishing node physical stats.
Traceback (most recent call last):
  File "/home/ray/anaconda3/lib/python3.10/site-packages/ray/dashboard/modules/reporter/reporter_agent.py", line 1971, in _run_loop
    json_payload = await loop.run_in_executor(
  File "/home/ray/anaconda3/lib/python3.10/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/ray/anaconda3/lib/python3.10/site-packages/ray/dashboard/modules/reporter/reporter_agent.py", line 1992, in _run_in_executor
    return asyncio.run(
  File "/home/ray/anaconda3/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/home/ray/anaconda3/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/home/ray/anaconda3/lib/python3.10/site-packages/ray/dashboard/modules/reporter/reporter_agent.py", line 2039, in _async_compose_stats_payload
    return self._generate_stats_payload(stats)
  File "/home/ray/anaconda3/lib/python3.10/site-packages/ray/dashboard/modules/reporter/reporter_agent.py", line 2076, in _generate_stats_payload
    parsed_stats = StatsPayload.parse_obj(stats_dict)
  File "/home/ray/anaconda3/lib/python3.10/site-packages/pydantic/main.py", line 1370, in parse_obj
    return cls.model_validate(obj)
  File "/home/ray/anaconda3/lib/python3.10/site-packages/pydantic/main.py", line 716, in model_validate
    return cls.__pydantic_validator__.validate_python(
pydantic_core._pydantic_core.ValidationError: 1 validation error for StatsPayload
workers.70.cmdline
  Input should be a valid list [type=list_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.12/v/list_type

The root cause is that psutil.Process.as_dict() returns None for any attribute whose getter raises AccessDenied/ZombieProcess (psutil's default ad_value is None). So a worker's cmdline can come back as None, but the StatsPayload schema (cmdline: List[str]) rejects.

[gemini-code-assist]

Code Review

This pull request introduces a Pydantic field validator to normalize None values in cmdline fields to empty lists across ProcessInfo and StatsPayload models, and adds a corresponding unit test. The reviewer pointed out that the new test will fail in minimal Ray installations where Pydantic is not installed, and suggested skipping the test in those environments.

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