Fix FE bulk update sending empty array

What does this MR do and why?

This MR fixes the frontend from sending an empty array for assignee_ids to the bulk issues REST API, since we should ideally send undefined because an empty array can be interpreted as "remove all existing assignees". Discussion in: https://gitlab.slack.com/archives/C72HPNV97/p1675782539856769

Screenshots or screen recordings

How to set up and validate locally

1. Visit a group issues page such as http://127.0.0.1:3000/groups/h5bp/-/issues
2. Click Edit issues to start bulk editing
3. Select the checkbox of an issue that has an assignee
4. In the sidebar, select any milestone
5. Update the issue, then confirm it still has an assignee after page reload

MR acceptance checklist

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