### What?
Ensure fields using a custom `toCSV` function that return `undefined`
are excluded from the exported CSV.
### Why?
Previously, when a `toCSV` function returned `undefined`, the field key
would still be added to the export row. This caused the column to appear
in the CSV output with an empty string value (`""`), leading to
unexpected results and failed assertions in tests expecting the field to
be truly omitted.
### How?
Updated the `flattenObject` utility to:
- Check if the value returned by a `toCSV` function is `undefined`
- Only assign the value to the export row if it is explicitly defined
- Applied this logic in all relevant paths (arrays, objects, primitives)
This change ensures that fields are only included in the CSV when a
meaningful value is returned.
751691aeaf