fix: deparse casts to single-field row types

[PHJ2000]

SELECT CAST(ROW(1) AS ROW(x BIGINT)) parses successfully, but toString() and StatementDeParser both dereference a null scalar type instead of rendering the one-element row definition.

Treat any nonempty columnDefinitions list as a row target in both serializers. Add a regression that checks the parsed field metadata and independently exercises both serialization paths, plus a TRY_CAST round trip. Existing multi-field and scalar cast coverage remains in the same test class.

Validation: the new regression fails on unmodified master with both NPEs; the same query also fails with the released 5.4 artifact. mvn verify completed the full suite (9,525 tests, 0 failures/errors, 25 skipped), PMD and Checkstyle. The subsequent spotless:check requested import sorting in the two touched production files; mvn spotless:apply spotless:check then passed.

Fixes #2756.