Resolve "Filter by `None`/`Any` for assignee_id in issues/mrs API"

What does this MR do?

Adds None and Any option for assignee_id in the Issues and Merge Requests API

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 #52384 (closed)