Update serialize.js to correct handling of relationships

[k8n]

Corrects serialization of one-to-many relationship, e.g. when serializing a structure like this:

{
    "id": "5",
    "title": "Welcome!",
    "type": "calendar-event",
    "participantCalendars": [
        {
            "id": "9",
            "type": "calendar",
            "name": "Rando",
            "color": "#b1a0c6",
        },
        {
            "id": "11",
            "type": "calendar",
            "name": "Bobert",
            "color": "#404040",
        }
    ],
    "calendar": {
        "id": "5",
        "type": "calendar",
        "name": "Froo",
        "color": "#ff0000",
    }
}

In addition, fetchja will now correctly serialize empty nested objects to null. For example, the following structure ...

{
    "id": "5",
    "title": "Welcome!",
    "type": "calendar-event",
    "calendar": {}
}

... will be serialized as follows:

{
    "id": "5",
    "type": "calendar-event",
    "attributes": {
        "title": "Welcome!",
    },
    "relationships" : {
        "calendar": { "data": null }
    }
}