Add None/Any options for assignee in search bar

What does this MR do?

Adds None and Any option for assignee in search bar

What are the relevant issue numbers?

This requires the API changes in #52384 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

Closes #52383 (closed)