fix(workbook): preserve falsey saved view comparison values

[rudycelekli]

Summary

Saved view comparisons currently convert 0 and false into an empty string. A persisted zero-score or false-result filter can therefore select a blank row instead of its matching row, and CSV export repeats the same wrong scope.

Use an explicit null fallback when normalizing the comparison value, matching the client’s existing String(value ?? "") behavior. List and export continue to share the same query.

Validation

• Native FastAPI/SQLite before: two persisted-view regressions fail, each returning the blank row instead of the matching row.
• After: 214 workbook tests passed, including numeric zero, canonical formula "false" results, string controls, all four comparison operators, and unchanged null/empty comparisons; CSV contents are checked as well.
• Frozen offline backend suite: 1,896 passed, 131 skipped, 3 deselected, with two unchanged warnings.
• PostgreSQL integrity and full frontend hosted gates are pending. No live provider or paid calls.

This changes comparison normalization only. Boolean controls use the existing formula result representation ("false"); raw JSON boolean column rendering is unchanged.

Prepared with AI assistance; source and native regression evidence were reviewed before submission. Signed-off commits included.