fix: prevent bare partition columns from inheriting the previous item's value

[fudianchn]

AI disclosure: this change was prepared with AI coding agents, reviewed and revised line by line by me.

What

A bare (dynamic) partition column no longer inherits the value expression of the preceding item in a mixed INSERT ... PARTITION (...) list; it keeps null again, as before #2323.

Why

With a static item followed by a bare one, the deparser output silently turned the dynamic column into a constant assignment, changing the statement's semantics for Hive mixed static/dynamic partition writes (#2500).

How

Partitions() declares one valueExpression for the whole production and the optional "=" Expression() unit leaves the previous item's value in place when skipped. The list loop arm now resets valueExpression = null before the optional unit, the same idiom already used by DeclareType, Alias and ColumnsNamesList for their list items. No AST, deparser or visitor change: Partition already renders a bare column when the value is null.

Root cause

Production-level valueExpression in Partitions() is shared across list items and was not reset when an item skipped the optional value (introduced with #2323).

Testing

• New InsertTest#testInsertMixedStaticAndDynamicPartitions covers the full [BUG] JSQLParser 5.4-SNAPSHOT : Hive : INSERT PARTITION bare column inherits the previous item's value #2500 combination table: static -> bare, bare -> static, static -> bare -> bare, bare -> static -> bare, and INSERT INTO with a numeric value. All wrong rows fail on master and pass with the change; the two correct rows are pinned as guards. Two mutants verified: removing the reset and misplacing it after the optional unit both turn the test red.
• Local run: gradlew test --tests InsertTest -> 79 passed.
• No jmh run: performance.sql contains no PARTITION clause, the changed action is not reached by the benchmark.

Verification of the original issue

Insert insert = (Insert) CCJSqlParserUtil.parse(
        "INSERT OVERWRITE TABLE t PARTITION (dtime = '20220403', region) SELECT code, region FROM src");
// before: dtime -> '20220403', region -> '20220403'  (wrong)
// after:  dtime -> '20220403', region -> null        (correct)

Fixes #2500

[manticore-projects]

Thank you much!