Fix assigning issues to users for Jira import

What does this MR do?

This is a similar fix as for labels (!30697 (merged)). We basically need to handle array attributes in the ImportIssueWorker as we use bulk insert.

The MR doesn't need a changelog entry, the problem is not event yet deployed on prod

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.