[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        | 
I don't think this will work if `$data` doesn't implement `\Traversable`