[Serializer] properly handle null data when denormalizing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10794
| License       | MIT
| Doc PR        | 
Yes, it does. PHP traverses the public properties of the object being passed. Actually, you can see that in one of the added tests.