Properly handle relations without data

[JaZo]

Description

If a relation does not have data (i.e. only links or meta), it should not be serialized as if it is empty (i.e. null or an empty array). This PR addresses that issue and also adds links and meta to the serialized array as those were missing.

Motivation and context

Fixes #77.

How has this been tested?

Tested with JSON provided in #77.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.