Add: "no universe set" filter to document list

### Steps to reproduce the behavior

1. Go to https://www.notebook.ai/documents
2. Click on "Filter by universe"

### Expected behavior

There should be an option "No universe set". (My current goal is to sort my documents by universe).

### Actual behavior

Only "All universes" and "universes that are currently used by documents" are seen.

Start at https://www.notebook.ai/documents and inspect the “Filter by universe” control. Add a “No universe set” option alongside the existing choices, and verify that it lists documents without a universe while preserving the current filtering options.

Written by the indexing model from the issue text.