Empty attribute values should be sorted consistently

[benel]

Description

What is the valuable outcome that cannot be achieved because of this bug?

For which stakeholder (people, role, project, domain) is it important?

Reproduction scenario

Steps to reproduce the behavior:

1. Go to the "vitraux" portfolio
2. Sort by spatial
3. Several items with no attribute value are among others (especially "SPSP ???").

Expected behavior

They should be either at the beginning or at the end of the list.

Workaround

Solution you found to avoid the problem.

Deliverables status

Phase 1

• [ ] Scenarios (Gherkin)
• [ ] Implementation strategy

Phase 2

• [ ] Acceptance tests (Capybara)
• [ ] Implementation