Attempting to remove a referenced object attribute of an existing object with import doesn't work

Issue Summary

Attempting to remove a referenced object attribute of an existing object with an import where the attribute value is left empty doesn't actually clear the value of that attribute after import. This only seems to happen with referenced attributes as text it can be observed that text attributes get cleared. 

Steps to Reproduce

1. Create a CSV/JSON import configuration that attempts to import referenced objects where the attributes all have values.
2. Set up object structure and configuration. 
3. Execute import. 
4. Re-attempt import with the referenced object attribute value cleared for one object.

Expected Results

The object is updated with referenced object attribute having an empty value.

Actual Results

The object is not updated and the referenced object attribute value remains the same.

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Workaround

Manually clear the attribute values of the relevant objects