[WIP] PIM-3309: Fix check on product unicity

This PR fix the issue #1657

| Q                    | A
| -------------------- | ---
| Bug fix?             | Yes
| New feature?         | No
| BC breaks?           | No
| CI currently passes? | Yes
| Tests pass?          | Yes
| Scenarios pass?      | Yes
| Checkstyle issues?*  | No
| PMD issues?**        | No
| Changelog updated?   | Yes
| Fixed tickets        | PIM-3309
| Doc PR               | No
BC break, if someone has overriden this service, the $this->productManager will be null and you'll try to call valueExists on a null object ;) please check if !== null before to call and make this argument mandatory when you'll merge on master