KIP-1040 — Improve handling of nullable values in InsertField, ExtractField, and other transformations

Accepted Kafka 3.9 Connect

Fixes several Kafka Connect SMTs (InsertField, ExtractField, SetSchemaMetadata, and others) to correctly propagate null field values when the JsonConverter is configured with replace.null.with.default=false. The SMT implementations created copies of Struct objects through internal paths that unconditionally substituted field defaults for null values, overriding the converter's explicit null-preservation setting.

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