fix(openapi-generator): fixes GlobalSettings

[cagliostro92]

This pull request aims to fix an issue I've noticed when adding a non-string value or key as a system property.
The issue surfaces during the invocation of CodeMojo#execute.
In such cases, GlobalSettings#log throws a ClassCastException as expected/partially documented here.

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[cagliostro92]

Hi @wing328, thanks to highlighting that. I'm going to fix it asap 👍🏻

[wing328]

can you please review the build failures when you've time?