[UX] Allow filtering by the value 'empty' in the grid

| Q                   | A
| ------------------- | ---
| Bug fix?            | N
| New feature?        | Y
| BC breaks?          | N
| Tests pass?         | Y
| Scenarios pass?     | ?
| Checkstyle issues?  | N
| Changelog updated?  | N
| Fixed tickets       | PIM-2426
| Doc PR              | ~

- [x] Text filter
- [x] Number filter
- [x] Price filter
- [x] Metric filter
- [x] Date filter
- [x] Boolean filter
- [x] Simple/multi-select ajax filter
- [ ] Simple/multi-select entity filter