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        | 
This method definitely expects an array as the code uses `$data[...]`. So, this fix does not really fix the whole issue, just part of it.

Isn't just about dealing with the special `null` value instead?