Merge branch feature/product-rest-services into develop

Added explicitly `nullable="false"` for the field `number`
Pull request is directly related to https://github.com/sulu-cmf/SuluProductAdvancedBundle/pull/4

- [ ] test coverage
- [ ] finish the code
- [x] gather feedback for my changes
- [ ] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| Tests pass?   | [none]
| Fixed tickets | [no]
| Doc           | [none]