[Serializer] Return an ArrayObject for empty collection objects when PRESERVE_EMPTY_OBJECTS is set #42730
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In #42619, we tried adding a BC layer for this behavior change, but when reviewing #42699, I figured out there is no possible BC layer: userland implementations (eg an entity returning an empty doctrine collection) won't extend ArrayObject because we ask them to do so.
I therefor propose to return an ArrayObject as of 5.4, relying on the intuition that this should not have much BC impact in practice.
/cc @lyrixx