PIM-2243 Fix variant group grid with mongo

| Q                   | A
| ------------------- | ---
| Bug fix?            | Y
| New feature?        | N
| BC breaks?          | N
| Tests pass?         | Y
| Scenarios pass?     | ?
| Checkstyle issues?  | N
| Changelog updated?  | N
| Fixed tickets       | PIM-2243
| Doc PR              |