Fix start error in load_persistent_executions

[olethanh]

Fix parse error that prevented Supervisor from starting when loading persistant executions, as it could not parse the gpu field

Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/opt/aleph-vm/aleph/vm/orchestrator/__main__.py", line 4, in <module>
    main()
  File "/opt/aleph-vm/aleph/vm/orchestrator/cli.py", line 379, in main
    supervisor.run()
  File "/opt/aleph-vm/aleph/vm/orchestrator/supervisor.py", line 184, in run
    asyncio.run(pool.load_persistent_executions())
  File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/opt/aleph-vm/aleph/vm/pool.py", line 252, in load_persistent_executions
    execution.gpus = parse_raw_as(List[HostGPU], saved_execution.gpus)
  File "pydantic/tools.py", line 74, in pydantic.tools.parse_raw_as
    obj = load_str_bytes(
  File "pydantic/parse.py", line 37, in pydantic.parse.load_str_bytes
    return json_loads(b)
  File "/usr/lib/python3.10/json/__init__.py", line 339, in loads
    raise TypeError(f'the JSON object must be str, bytes or bytearray, '
TypeError: the JSON object must be str, bytes or bytearray, not NoneType

Explain what problem this PR is resolving

Related ClickUp, GitHub or Jira tickets : ALEPH-XXX

Self proofreading checklist

• The new code clear, easy to read and well commented.
• New code does not duplicate the functions of builtin or popular libraries.
• An LLM was used to review the new code and look for simplifications.
• New classes and functions contain docstrings explaining what they provide.
• All new code is covered by relevant tests.
• Documentation has been updated regarding these changes.
• Dependencies update in the project.toml have been mirrored in the Debian package build script packaging/Makefile

Changes

Explain the changes that were made. The idea is not to list exhaustively all the changes made (GitHub already provides a full diff), but to help the reviewers better understand:

• which specific file changes go together, e.g: when creating a table in the front-end, there usually is a config file that goes with it
• the reasoning behind some changes, e.g: deleted files because they are now redundant
• the behaviour to expect, e.g: tooltip has purple background color because the client likes it so, changed a key in the API response to be consistent with other endpoints

How to test

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Notes

Things that the reviewers should know: known bugs that are out of the scope of the PR, other trade-offs that were made.
If the PR depends on a PR in another repo, or merges into another PR (i.o. main), it should also be mentioned here

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.32%. Comparing base (07a7adb) to head (2840128).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/pool.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #759   +/-   ##
=======================================
  Coverage   63.32%   63.32%           
=======================================
  Files          77       77           
  Lines        6844     6844           
  Branches      568      568           
=======================================
  Hits         4334     4334           
  Misses       2329     2329           
  Partials      181      181           

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