Fix ignoreEmpty=false to keep empty header columns

[Rithick574]

Ensure missing trailing cells are treated as empty strings, and add a regression test for headers with empty column values.

[Rithick574]

Issue: When CSV rows omit trailing empty cells, the parser only iterates row.length, so header columns beyond that are never emitted. This makes columns with all-empty values disappear even with ignoreEmpty: false.

Fix: When ignoreEmpty is false, we iterate up to headers.length and treat missing cells as empty strings. This preserves empty header columns while still skipping entirely blank lines. Added a regression test (dataEmptyColumns) to cover the scenario.