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

[ityuhui]

Hi @wing328 @zhemant @michelealbano

This PR is similar to #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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[ityuhui]

# make
Scanning dependencies of target openapi_petstore
[  6%] Building C object CMakeFiles/openapi_petstore.dir/src/list.c.o
[ 13%] Building C object CMakeFiles/openapi_petstore.dir/src/apiKey.c.o
[ 20%] Building C object CMakeFiles/openapi_petstore.dir/src/apiClient.c.o
[ 26%] Building C object CMakeFiles/openapi_petstore.dir/external/cJSON.c.o
[ 33%] Building C object CMakeFiles/openapi_petstore.dir/model/object.c.o
[ 40%] Building C object CMakeFiles/openapi_petstore.dir/model/api_response.c.o
[ 46%] Building C object CMakeFiles/openapi_petstore.dir/model/category.c.o
[ 53%] Building C object CMakeFiles/openapi_petstore.dir/model/order.c.o
[ 60%] Building C object CMakeFiles/openapi_petstore.dir/model/pet.c.o
[ 66%] Building C object CMakeFiles/openapi_petstore.dir/model/tag.c.o
[ 73%] Building C object CMakeFiles/openapi_petstore.dir/model/user.c.o
[ 80%] Building C object CMakeFiles/openapi_petstore.dir/api/PetAPI.c.o
[ 86%] Building C object CMakeFiles/openapi_petstore.dir/api/StoreAPI.c.o
[ 93%] Building C object CMakeFiles/openapi_petstore.dir/api/UserAPI.c.o
[100%] Linking C shared library libopenapi_petstore.so
[100%] Built target openapi_petstore

[wing328]

LGTM