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Add a configuration to JsonConverter:

Name: replace.optional.null.with.default
Description: Whether to replace fields that have a default value and that are null to the default value. When set to true, the default value is used, otherwise null is used.
Type: Boolean
Default: true

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Rejected Alternatives

1. Always take null on an optional a null field which has default value. It will break the compatibility and only cover partial cases.
2. Add the configuration to ConverterConfig