Unset relation when null (singular) or empty array (plural)

[JaZo]

Description

You can now unset a relation using the ItemHydrator by passing null (singular relation) or [] (plural relation).

Motivation and context

The ItemHydrator currently hydrates the relation with an item with id null for singular relations or no relation at all for plural relations.

Closes: #66

How has this been tested?

Tested with new unit tests.

Types of changes

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• 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.