Do not fill type attribute in unmapped items in morph(to) relationships

[JaZo]

Description

I changed the ItemHydrator to omit the 'type' attribute when filling the attributes for an unmapped item.

Motivation and context

Without this fix, this resulted in items with 'type' attributes, which violates the spec.: https://jsonapi.org/format/#document-resource-object-fields

How has this been tested?

Tested with new 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.