[FLINK-36561][table] Fix ResultSet.wasNull() not reflecting per-column null values in JDBC driver

[Au-Miner]

What is the purpose of the change

Fix ResultSet.wasNull() in the Flink JDBC driver, which previously only tracked whether the whole row was null and so always returned false for individual null columns — violating the JDBC contract that wasNull() must reflect the value most recently read.

Brief change log

• Update each column getter in FlinkResultSet (getString, getBoolean, getByte, getShort, getInt, getLong, getFloat, getDouble, getBytes, getObject, getBigDecimal) to set the wasNull flag from currentRow.isNullAt(...) (or the equivalent null check on the returned value) before returning. Label-based overloads and getDate/getTime/getTimestamp inherit the fix by delegation.

Verifying this change

Added FlinkResultSetTest#testWasNullReflectsPerColumnNullness, which reads a row of alternating null and non-null columns across BOOLEAN/TINYINT/SMALLINT/INT/BIGINT/FLOAT/DOUBLE/DECIMAL/STRING/BYTES/MAP and asserts that wasNull() flips to true after each null read and back to false after each non-null read.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no

[flinkbot]

CI report:

• b11dcb6 Azure: SUCCESS

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