PIM-2420 : Fix import of product associations (MongoDB support)

| Q                   | A
| ------------------- | ---
| Bug fix?            | Y
| New feature?        | N
| BC breaks?          | 
| Tests pass?         |
| Scenarios pass?     |
| Checkstyle issues?  |
| Changelog updated?  |
| Fixed tickets       |
| Doc PR              |