fix: use getWithoutDefault to avoid Connect schema default for null values

[waterWang]

Fixes #17652

Problem

Kafka Connect's Struct.get(Field) returns the field's schema defaultValue when the stored value is null, without checking isOptional() (longstanding Connect behavior, see KAFKA-8713). The Iceberg sink and its bundled SMTs read field values with get() in several places, so an explicit NULL in the source ends up written to the Iceberg table as the column's default value.

The most common trigger is Debezium CDC: for any column that is nullable with a non-NULL default (e.g. MySQL VARCHAR(255) NULL DEFAULT ''), every explicitly NULL value is replaced with the default.

Fix

Replace Struct.get(Field) with Struct.getWithoutDefault(String) at all affected call sites. getWithoutDefault returns the actual stored value without applying the schema default, so null values are preserved as null.

Changes

• DebeziumTransform.java — copy loop in applyWithSchema
• KafkaMetadataTransform.java — copy loop in applyWithSchema
• CopyValue.java — copy loop and single-field copy in applyWithSchema
• RecordConverter.java — field reads in structToIceberg, collectFieldNames, and structToVariant

[uros-b]

#17653 seems to be a strict superset of these changes, and it seems that it's well in progress - cc @moggaa