Fix unassigned checkmark

What does this MR do?

• Fixes regression with selecting Unassigned in the Assignee dropdown when filtering Issuables

Are there points in the code the reviewer needs to double check?

Make sure there are no unwanted side effects

Why was this MR needed?

Fixes regression

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31843 (closed)