Matrix is broken when using translations

[magicznyleszek]

Description

Having a matrix question and translations generates an unnecessary null translation when saving in Form Builder.

Steps to Reproduce

1. Create new form using Form Builder
2. Define a new question using "Question Matrix" (you can leave default values)
3. Save and close
4. In Manage Languages modal define default language (e.g. "English (en)") and close the modal
5. Open form in Form Builder
6. Don't edit anything, just hit Save and close
7. Observe Languages list is now "English (en), Unnamed language"

Expected behavior

The null language should not be added.

Additional details

Observed on https://kf.kobotoolbox.org/.

My guess would be to check out the nullifyTranslations/unnullifyTranslations flow and what it does to the matrix type. Probably Matrix has some custom hack on its own.