PIM-2975: Fix normalization of decimal attributes for MongoDB

Decimal product values where normalized as strings.

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | -
| Doc PR               | -
Not sure if you shouldn't throw an exception, as the normalizer will certainly fail because it's not an object.