entities with 'und' as their language get skipped by static export

Problem/Motivation

My site's nodes had 'und' as their language, probably because of a glitch in the migration from d7.

The tome static process skips them completely, because it iterates over

    $langcodes = array_keys($this->languageManager->getLanguages());

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes