Fix NRE in mock debug log for native commands

[fflaten]

PR Summary

Fixes NRE using mock debug logging when Should-Invoke with -ParameterFilter is used for a mocked native application. Caused by variable conflict-logging looking for CommandMetaData which doesn't exist for external commands = N/A logging.

Fix #2479

Backport to 5.6.0

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• [ ] Tests are added/update (if required)
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[fflaten]

While at it, let's save AST-processing for parameter-less functions as well.

[fflaten]

/backport to rel/5.6.x

[github-actions[bot]]

Started backporting to rel/5.6.x: https://github.com/pester/Pester/actions/runs/9177190930

[nohwnd]

/azp run

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