test: guard against null values in custom toCSV functions (#12938)

### 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.

test/plugin-import-export/collections/Pages.ts

@@ -44,9 +44,11 @@ export const Pages: CollectionConfig = {
       relationTo: 'users',
       custom: {
         'plugin-import-export': {
-          toCSV: ({ value, columnName, row, siblingDoc, doc }) => {
+          toCSV: ({ value, columnName, row }) => {
-            row[`${columnName}_id`] = value.id
+            if (value && typeof value === 'object' && 'id' in value && 'email' in value) {
-            row[`${columnName}_email`] = value.email
+              row[`${columnName}_id`] = (value as { id: number | string }).id
+              row[`${columnName}_email`] = (value as { email: string }).email
+            }
           },
         },
       },