[C][Client] Set the default value for the null json

[ityuhui]

@wing328 @zhemant @michelealbano

The PR #12557 adds the support for the free-form object. When it is merged to the downstream project https://github.com/kubernetes-client/c, I received a comment kubernetes-client/c#128 (comment) that the default value for the null json should be "null" other than "{}"

cc @harlequin-tech

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[ityuhui]

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