Allow sending the `null` value where it has a specific meaning

[swallez]

Description

Some fields in the settings API handle null values differently from missing values: sending null resets the settings to their default value. This is currently not supported in the Java API client.

See:

• https://discuss.elastic.co/t/java-api-client-how-to-reset-index-settings-value/329059 where this was reported
• Represent "meaningful null" elasticsearch-specification#2049 to capture this in the API specification

[l-trotta]

closed with https://github.com/elastic/elastic-client-generator-java/pull/13