Fix editable key columns after clearing legacy names

[minleejae]

Purpose

Allow callers to clear an index or constraint's legacy column names and then repopulate its structured key list. Today key.setColumnsNames(null) followed by addColumns(...) throws UnsupportedOperationException.

Current design (As-is)

The shared KeyColumnSource.setColumnsNames(null) installs Collections.emptyList(). Index, primary/unique key, foreign key and exclusion constraint nodes retain that list. Their fluent appenders expect an editable list, so null-clear behaves differently from an empty-name replacement.

Target design (To-be)

Use an empty ArrayList in the shared null-clear path. Both null and empty name lists clear the keys and leave them editable through getColumns() and the existing addColumns overloads. The legacy accessor still returns a rendered snapshot; replacing names still creates opaque keys and discards prior element-specific options. Indexes and constraints remain separate types.

key.setColumnsNames(null);
key.getColumns().add(new KeyElement("id"));

Validation

• Regression coverage exercises indexes, primary keys, unique keys, foreign keys and exclusion constraints, with null/empty clears, collection/varargs appenders and direct structured-list edits.
• Both toString() and StatementDeParser preserve the restored SQL and key options; rendered statements reparse successfully.
• Default PMD complexity and field-placement rules: zero violations for the changed shared interface.
• Java 17 offline Gradle spotlessApply check: 10,045 tests, 10,020 passed, 25 skipped, zero failures/errors. Grammar, Checkstyle, PMD, SpotBugs and coverage checks pass.

Integration validation

Applied #2770, #2771 and all four follow-ups (#2775–#2778) together on master (066f581e), resolving overlapping validator edits while preserving their respective behavior. The integrated build passes Java 17 offline Gradle spotlessApply check: 10,141 tests, 10,116 passed, 25 skipped, zero failures/errors. The combined ALTER validator has zero findings under the default PMD complexity and field-placement rules.

Replayed 882 previously DB-validated MySQL/PostgreSQL DDL fixtures: 875 pass and the same seven existing parse gaps remain. SQL output, reparsing and AST node-type sets match the preceding integration baseline, with no new differences. This replay did not rerun the databases.

The published head passes grammar checks, Gradle and Maven on Ubuntu/Windows/macOS, and Codacy. Deployment-only jobs are skipped as expected for a PR.