Possibly not necessary warning when exporting top-level taxonomy terms [#3622562]

Problem/Motivation

I was playing around with exporting an article node with 3 new tag terms using

dr content:export node 5043 --with-dependencies --dir=../recipes/academytwo_recipe_test/content

While this command was ultimately successful, it did output the following warning 3 times (once for each new term I added to the node's Tags field):

[warning] Failed to export reference to taxonomy term 0 referenced by Term Parents on taxonomy term batman because the referenced taxonomy term does not exist.

This warning message doesn't really make sense if the term being exported is a top-level term.

Steps to reproduce

Create a node with one or more new top level terms. Export as shown above.

Issue fork drupal-3622562

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