fix(csv-stringify): allow empty string column keys

[xiehuanyi]

Array column definitions reject key: "" as missing, although empty-string columns and object-form column mappings already select that valid record property.

Allow an empty string as an explicit key while preserving missing-key validation. Both explicit and default headers now serialize the empty-named field.

Validation:

• Two regressions failed before and pass after.
• Full csv-stringify suite: 234 passed, 1 existing pending test; TypeScript passed.
• Package build and changed-file ESLint passed.

Prepared with AI assistance. Source changes only; generated bundles were validated locally.

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