[WIP] PIM-2494 : update normalizers to remove null values from MongoDB normalized data

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