Resolve "Search bar rendering `none` and `any` as users"

What does this MR do?

Fix rendering of none and any tokens in the assignee filter.

Includes capitalized none and any as special values.

This is a follow-up to !23846 (merged) which caused a regression.

What are the relevant issue numbers?

#57692 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• 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.
• Security reports checked/validated by reviewer

Closes #57692 (closed)