Cope with null relationships

[pdw-mb]

I have some data with a nullable relationship. This results in API data like this:

{
    "relationships": {
        "language": {
            "data": null
        }
    }
}

This is valid according to the JSON API spec, but was causing an exception in json-api-merge when trying to resolve this relationship. This PR fixes this, although perhaps there is a more elegant way to do this not-null check in Ramda?

[char0n]

Hi @pdw-mb,

Thanks for the PR, and opening this up. I've read the spec, and you're absolutely right. What we want to do is to only pick relationships which data prop present and where prop !== null. I'll create a superseding PR with the change and appropriate tests.

[char0n]

This PR was superseded by #909. Thanks again @pdw-mb!