Persist empty relation as null instead of omitting it from the ES document

[limenet]

Given

$this->relationAttributesWithDefault($element, [
    ProductIndex::ATTRIBUTE_VARIETY => Product::FIELD_VARIETY,
]),

where variety is a one-to-one/many relation,

the resulting Elasticsearch when variety is empty is

{
}

and when variety points to object ID 91

{
  "variety": 91
}

.

This PR changes the case where the relation is empty to

{
  "variety": null
}

in order to make the behavior consistent with other methods in this trait. Hence this is considered a bugfix and not a breaking change.

---

Resolve #77