fix(client): add additional checks for the timeout

[srivatsa-cfp]

• Check if the value of token, url and timeout is null and return if they are null
• Check if the value of timeout is negative, if it's negative return.
• If this checks are not needed and only OpenAiService(final String token) is exposed, we should make OpenAiService(final String token, final String baseUrl, final Duration timeout) as private.