[C][Client] Do not put the invalid value of the enum to a JSON structure

Created by: ityuhui

The PR https://github.com/OpenAPITools/openapi-generator/pull/5477 supports the data type enum e.g.

typedef enum  { 
    kubernetes_v1_container_IMAGEPULLPOLICY_NULL = 0, 
    kubernetes_v1_container_IMAGEPULLPOLICY_Always, 
    kubernetes_v1_container_IMAGEPULLPOLICY_IfNotPresent, 
    kubernetes_v1_container_IMAGEPULLPOLICY_Never 
} kubernetes_v1_container_IMAGEPULLPOLICY_e;

And the default value is kubernetes_v1_container_IMAGEPULLPOLICY_NULL if the enum data does not appear in json string at all.

Actually kubernetes_v1_container_IMAGEPULLPOLICY_NULL is invalid to API server because it is not defined in OpenAPI Spec. So it should not be passed to API server.

But in the function *_convertToJSON, the current code puts the value to the JSON structure (which will be passed to the API server) whether or not the value is valid.

This PR will fix this issue. When the value of enum is kubernetes_v1_container_IMAGEPULLPOLICY_NULL, it will be ignored by *_convertToJSON

@wing328 @zhemant @michelealbano

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