Null references in dependencies

Problem/Motivation

In some cases export contains references to a null objects, e.g. when referring to a parent of a zero-level taxonomy term.
Later, when import occurs, this results in a "Trying to access array offset on value of type null" warning, which generally passes silently except for some specific cases such as test env setup for PHPUnit.

Steps to reproduce

Create and export taxonomy term, try importing it on another env(or after deleting, so that import actually works).

Proposed resolution

Check contents of the $decoded['_embedded'] items before using them.