Allow null values in content mapper if node property not exists

Allow null values in content mapper if node property not exists:

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none