Empty belongsTo relationship not passed back in correct JSONAPI format

[nickschot]

Currently an empty belongsTo relationship looks like this in the server response:

...
relationships: {
    user: null
}
...

While JSON API states the following format must be followed:

...
relationships: {
    user: {
        data: null
    }
}
...

The only difference with hasMany relationships is that those must (and do) have a data: [] instead of data: null.

Relevant literature: http://jsonapi.org/format/#document-resource-object-relationships
It also states there that a relationship response must contain at least one of data, links or meta.

I have not checked if the server handles this correctly when sending a request.

[nickschot]

Culprit seems to be in packages/serializer/index.js from line 628. The else if(related && related.id) fails when related is null and it then just returns null. I guess the format is also wrong when a belongsTo relationship is found as no {data: ...} is present within the else if either.