Adjust the logic to set the correct organization ID for the new records for `vulnerability_export_parts`

Records in this table is associated with vulnerability_exports which can be associated with a project, group, or nothing. There are foreign keys to projects and namespaces but those can be nullable.

We added organization to the vulnerability_export_parts table and then set organization_id as sharding key as part of Implement very large file export mechanism (!152517 - merged) • Gregory Havenga • 17.1.

In this issue we will adjust the logic to set the correct organization ID for the new records for vulnerability_export_parts.