### What?
Fixes a crash when exporting documents to CSV if a custom `toCSV`
function tries to access properties on a `null` value.
### Why?
In some cases (especially with Postgres), fields like relationships may
be explicitly `null` if unset. Custom `toCSV` functions that assume the
value is always defined would throw a `TypeError` when attempting to
access nested properties like `value.id`.
### How?
Added a null check in the custom `toCSV` implementation for
`customRelationship`, ensuring the field is an object before accessing
its properties.
This prevents the export from failing and makes custom field transforms
more resilient to missing or optional values.
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