fix(csv-stringify): quote empty strings returned by casts

[sunlishuo25]

When a cast changes a field to an empty string, quoted_match: /^$/ and quoted_string: true omit the quotes:

stringify([[' ']], {
  quoted_match: /^$/,
  cast: { string: value => value.trim() }
});
// before: '\n'; after: '""\n'

Select the empty-string branch using the cast value, while keeping quoted_string limited to original string fields. Tests cover callback and sync APIs, field-level options, null/undefined, non-string fields, and quoted_empty: false.

The default boolean cast returns '' for false, so false now also matches quoted_match: /^$/. Null and undefined stay unquoted with that option.

Validation on Node 24.16.0:

• Four regressions fail before the fix; all 19 focused tests pass afterward.
• npm test --workspace csv-stringify -- --no-parallel: TypeScript passed, 237 tests passed, 1 existing pending test.
• Stream pipeline through csv-parse, ESLint and Prettier passed.
• Isolated package build passed; packed CommonJS and actual-browser ESM/IIFE/UMD sync and callback entries passed.
• Independent distribution checks passed 36 CJS and 72 browser assertions, plus 6 environment checks.

Node 20/22 and the wider browser matrix were not run locally. Generated files are omitted, following recent source-fix PRs and the package's release build scripts.

Developed with GPT assistance.