[C][Client] Fix default enum value being -1

[bookerdj]

fix #17513

This changes the template for enums to use their NULL value as a default. I have also regenerated the sample code.
This is potentially more breaking than my previous change but without the fix the code doesn't work.

@zhemant
@ityuhui
@michelealbano

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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

[zhemant]

Thank you for the changes. Looks good to me

[bookerdj]

Ok, I think I've fixed the commit so it's linked to my account.

[ityuhui]

LGTM