Problem/Motivation
The \Drupal\migrate_drupal\Plugin\migrate\process\NodeCompleteNodeTranslationLookup process plugin return valid values for a non translated node and it should not. This was discovered while working on #3082211: Migrate UI upgrade tests should provide the complete log.
The process plugin is designed to convert a lookup on the node_complete migration to a lookup on the node_translation migration. But it will return a valid value for a non-translation, if it exists in the map table. It should not do that. It should be mimicking the node_translation migration which, of course, only has mappings for translated nodes.
Steps to reproduce
Proposed resolution
Change the process plugin to return NULL if the source language is 'und'
Add a test
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