Add extra verifications for links

[JaZo]

Description

• Self and related links can no longer be null.
• Some links objects MUST now have certain links if a links object is present:
  Error: about
  Relationship: self or related

Motivation and context

According to the spec, self and related links can not be null, only pagination links can be.

Spec for error: https://jsonapi.org/format/#error-objects
Spec for relationship: https://jsonapi.org/format/#document-resource-object-relationships

How has this been tested?

Tested with new and updated unit tests.

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.