fix(stringify): reject invalid values in cast option objects

[kirisue]

csv-stringify silently writes an empty field when a custom cast returns an options object whose value is 0 or false:

stringify([['input']], { cast: { string: () => ({ value: 0 }) } })
// before: '\n'; after: throws Invalid Casting Value

The existing type guard allows only strings, null and undefined, but an unrelated if (err) prevents it from reporting the invalid value. Remove that condition so invalid values are rejected before formatting. Valid string, null and undefined values retain their existing behavior.

Add regression cases for 0 and false in both the callback and synchronous public APIs, plus a control covering the allowed value types. Unsupported truthy values are also rejected by this earlier guard and now receive its Invalid Casting Value message rather than the later Formatter message.

Validation on Node.js 24.19.0:

• The four invalid-value regressions fail before the source change and pass afterward.
• Focused cast suite: 7 passed.
• npm test --workspace csv-stringify: TypeScript check passed; 232 tests passed, 1 pre-existing pending test.
• ESLint and Prettier on both changed files: passed.
• No generated distribution, dependency or lockfile changes.

Prepared with OpenAI Codex; the listed tests were executed locally.

[wdavidw]

Hi @kirisue, could you provide me with the permission to push into your fork. I wish to update the tests.

[kirisue]

Hi @wdavidw, I've enabled “Allow edits by maintainers” for this PR. You should now be able to push the test updates directly to the branch. Thanks for taking a look!

[wdavidw]

Thank you @kirisue for your involvement.