D6 MySQL to D8.6.x PgSQL Migration
When using source plugin d6_taxonomy_vocabulary_per_type, if the Internationalization module was not enabled in D6 and there is no variable set 'i18ntaxonomy_vocabulary', the Migrate source outputs a harmless but noisy error.
In the Migrate source, prepareRow defaults $i17ntaxonomy_vocab to NULL if the variable is unset, then passes it to in_array.
We could either default it to an empty array (which handles the case of it being unset), or check the input with is_array() (which also handles cases where it's set to an unexpected value such as an integer).