[PR follow up] Make getConfigurationClient in application_config has optional parameter

[tianleh]

Inspired by @bandinib-amzn 's comment https://github.com/opensearch-project/OpenSearch-Dashboards/pull/5641/files#r1518382217 , create this PR to track the improvement of the getConfigurationClient function https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/application_config/server/plugin.ts#L53

The parameter is actually optional when there is an external configuration. We could make it clear so that users of this function are aware of this and may not need to pass an explicit null.