If none of the nested object's properties are part of serialization groups, array is returned instead of null

[ostrolucky]

API Platform version(s) affected: 3.2.18

Description
If object is part of the correct serialization group, but nested object doesn't have this group assigned, nested object is serialized as empty array. Instead, I would expect exception to be thrown or null returned instead of array

How to reproduce
Given code

class Foo {
  public __construct(#[Groups('groupA')] public Bar $bar) {}
}
class Bar {
  public __construct (#[Groups('groupB')] public string $baz) {};
}

And return new Foo(new Bar('baz')); with groupA as serialization group, APIP will serialize it like so {"foo": {"bar": []}}. Instead, I expect it will serialize it like {"foo": {"bar": null}}, since serializing this as array breaks OpenAPI contract

Possible Solution
Throw exception, or return null