[FLINK-38647][CDC] Improve error message for null before/after struct…

[BreadS00]

When PostgreSQL table uses default REPLICA IDENTITY DEFAULT or REPLICA IDENTITY PRIMARY KEY, the logical decoding plugin (pgoutput) may not provide complete before-data for UPDATE events. This causes the before struct to be null in Debezium events, leading to an unhelpful NullPointerException.

[Copilot]

Pull request overview

Improves diagnostics when Debezium before/after payload structs are unexpectedly null (common with PostgreSQL REPLICA IDENTITY DEFAULT/PRIMARY KEY), replacing an unhelpful downstream NullPointerException with a targeted message and an optional connector-specific hint.

Changes:

• Add explicit null checks for before/after structs in DebeziumEventDeserializationSchema, throwing with a clearer message.
• Introduce an overridable getNullBeforeDataHint() hook for connector-specific guidance.
• Provide a PostgreSQL-specific hint suggesting REPLICA IDENTITY FULL.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
flink-cdc-connect/flink-cdc-source-connectors/flink-connector-debezium/src/main/java/org/apache/flink/cdc/debezium/event/DebeziumEventDeserializationSchema.java	Adds null checks for before/after and a hook for connector-specific hint text.
flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-postgres/src/main/java/org/apache/flink/cdc/connectors/postgres/source/PostgresEventDeserializer.java	Overrides the hint hook to provide a PostgreSQL-specific remediation suggestion.

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