Handling of null data

[TeamNautilus]

As stated in the JSON API specs a resource linkage object may have these values:

Resource linkage MUST be represented as one of the following:

    null for empty to-one relationships.
    an empty array ([]) for empty to-many relationships.
    a single resource identifier object for non-empty to-one relationships.
    an array of resource identifier objects for non-empty to-many relationships.

https://jsonapi.org/format/#document-resource-object-linkage

After the json-api-vanilla parsing is return an Object instead.

To reproduce:

• Change the example.json:84 from
  "data": { "type": "people", "id": "9" }
  to
  "data": null

• Add the following test:

it "should handle null data" do
   expect(doc.data.first.comments.last.author).to be_nil
end

Result:

     expected: nil
     got: #<Object:0x0000557bbcd4bd08>

We have noticed the fallback creation of an object here:

ref = ref || Object.new

Removing that line make all specs green but maybe it will results in other "untested" issues.