I've another question, while we can define default values for schema in JsonSchema -- how can these default values be used in the streams, is there a way where defined default from schema can be used in record if the record is null?

Not currently 🙂. But log an issue in the SDK: https://github.com/meltano/sdk/issues/new?assignees=meltano%2Fengineering&labels=k[…]%2FSDK&projects=&template=feature.yml&title=feat%3A+%3Ctitle%3E