[C][Client] Allow cJSON_IsNull() for a string if it is not mandatory

Created by: ityuhui

Hi @wing328 @zhemant @michelealbano

This PR is similar to https://github.com/OpenAPITools/openapi-generator/pull/13884

Sometimes, the value of string data type in the JSON/YAML string is null

e.g.

{
  "kind": "NodeList",
  "apiVersion": "v1",
  "metadata": {
    "continue": ""
  },
  "items": [
    {
      "kind": "Node",
 ...
      "status": {
 ...
        "phase": null  <---- Here !
      }
    }
  ]
}

Currently openapi-generator/c-libcurl doesn't allow this case, so parsing JSON strings into cJSON objects will fail.

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