PIM-3545 Allow setting values to null/empty in product value setters

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | N
| Tests pass?          | Y
| Scenarios pass?      | running
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | N
| Fixed tickets        | ~
| DB schema updated?   | N
| Migration script?    | N
| Doc PR               | ~
file can be null when changin from 'haz file' to 'no haz file'. Locale and scope can be null anyway, this is just to have optional parameters nicely at the end.