[C][Client] Check cJSON_IsNull when the data type is datetime

Created by: ityuhui

Sometimes, the value of date-time data type in the JSON/YAML string is null e.g.

- apiVersion: v1
  count: 1
  eventTime: null  <----- here !
  firstTimestamp: "2022-11-02T09:18:07Z"
  kind: Event
  lastTimestamp: "2022-11-02T09:18:07Z"
  ...

Currently openapi-generator/c-libcurl cannot handle this case when parsing JSON strings into cJSON objects.

e.g. https://github.com/kubernetes-client/c/issues/144

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