Set selected Assignee in filter from params

What does this MR do?

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

Why was this MR needed?

• When Unassigned was selected, the dropdown would display Any Assignee as the active selection

Screenshots (if relevant)

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)