Fix: AttributeError when self.vm == None

[hoh]

Fixes the following error, reported by node operators.

task: <Task finished name='Task-1' coro=<_run_app() done, defined at /usr/lib/python3/dist-packages/aiohttp/web.py:287> exception=AttributeError("'NoneType' object has no attribute 'to_dict'")>
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/aiohttp/web.py", line 431, in _run_app
    await asyncio.sleep(delay)
  File "/usr/lib/python3.10/asyncio/tasks.py", line 605, in sleep
    return await future
asyncio.exceptions.CancelledError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/aiohttp/web.py", line 433, in _run_app
    await runner.cleanup()
  File "/opt/aleph-vm/aleph/vm/models.py", line 234, in stop
    await self.record_usage()
  File "/opt/aleph-vm/aleph/vm/models.py", line 275, in record_usage
    pid_info = self.vm.to_dict()
AttributeError: 'NoneType' object has no attribute 'to_dict'

[github-actions]

Failed to retrieve llama text: HTTPConnectionPool(host='46.255.204.205', port=5100): Max retries exceeded with url: /completion (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f5a66264490>: Failed to establish a new connection: [Errno 111] Connection refused'))