op-mode: T6367: fix "force commit-archive" TypeError (backport #3489)

[mergify]

Change Summary

/usr/bin/config-mgmt requires an argument OR to be symbolically linked to *commit-revision or *commit-archive, for which it interprets argv[0] through the useful trickery:

https://github.com/vyos/vyos-1x/blob/current/python/vyos/config_mgmt.py#L693-L700

This fixes:

Traceback (most recent call last):
  File "/usr/bin/config-mgmt", line 33, in <module>
    sys.exit(load_entry_point('vyos==1.3.0', 'console_scripts', 'config-mgmt')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/vyos/config_mgmt.py", line 746, in run
    func = getattr(config_mgmt, args['subcommand'])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: attribute name must be string, not 'NoneType'

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6367

Related PR(s)

Component(s) name

op-mode

Proposed changes

How to test

vyos@vyos:~$ force commit-archive

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3489 done by [Mergify](https://mergify.com).